This is going to be a pretty general basic overview, but in the US there are basically three classes of Federal Land that is "open" to the public. There are other classes like military bases, conservation areas, and the like, but that is going further down the rabbit whole.
The first is National Parks, and are basically the most beautiful or not worth examples of a given chunk of country. They have the most stringent regulations, oversight, and the highest concentrations of people. Until a few years back firearms were not allowed in National Parks at all. The law/regulations changed and now they are allowed, but as has been pointed out not in the Federally Owned Buildings. Of course this applies to all Federally Owned Buildings, so it applies to all these categories. In regards to firearms in Parks, it is best to treat them like cities and concealed is the name of the game. I am not aware of any parks that allow hunting. The total amount of Park Lands is pretty limited in comparison to the next two categories. There is also generally either of the two next to every park and if you want to avoid the people and regulations you can generally see stuff as beautiful or spectacular without venturing into the park.
In theory the next two categories are divided roughly between range (cattle) and forest (logging) uses, but the reality is a bit different.
National Forests are big chunks of fed land that are beautiful but not the stand out pieces, and are so designated to preserve natural resources. There are wildernesses as part of forests that have restricted uses (simple description no wheels no mechanical machines) in order to keep them wild, undeveloped and an example of how things were before. So within forests you will find different levels of restriction and oversight. Generally, most forests will have a popular trail or even several depending on size and roads and campgrounds. All of those will see heavy use, and for the most part the rest of the forest will be empty and unused outside hunting season. That is why on backpacking trips or even hikes we seldom see folks once we are away from the roads. There is a tremendous amount of variation in forests based on parts of the part of the country, size, and use areas. In theory each forest is managed to the best benefit of all the different user groups with an eye on preserving things. In some I feel pretty comfortable carrying whatever I want away from the popular trails and road/campground areas since it is unlikely I am going to see anyone. As far as actual firearm restrictions in general there are going to be no shooting areas (campgrounds for instance), but other then that you can pretty much roll subject to state laws and fed laws (an illegal gun is an illegal gun). During hunting seasons you better have tags and be in compliance with game laws regarding firearms. There is always going to be a percentages of folks who are simply uncomfortable see guns. In some parts of the country, mostly back east and along the west coast, those percentages are very high just due to local views. In those areas being more circumspect is better. However, in the majority of the west no one is going to take it amiss, unless you are acting the fool, or to commandoed out. It used to be that ARs were looked on with a bit of suspicion, but they have become so prevalent that is changing slowly and surely as they are becoming the replacement for the traditional 30/30.
The last category is the Bureau of Land Management (BLM), and yes I had a lot of confusion caused by that one early on, which in theory is land best suited for mineral extraction and range use. The problem is that Forests are used for range use and mineral extraction as well. The reality is that it was at the time considered crap land that no one wanted. Heck the folks using it for the most part didn't want ownership, when they could have gotten it. That has changed in more recent years with the price of oil and the evolution and growth of some outdoor user groups like the ohv guys and the mountain bike guys. Traditionally BLM has been very loosely administered and pretty much anything went. That is how most of the trails and roads in the Moab area for instance were developed. It was a beautiful amazing area and people went there and drove around, checked things out, and then started challenging themselves with harder and harder trails. Kind of a I wonder if I could thing. As the number of users has gone up so has the regulation and ironically in a lot of areas there is a strong push being made by mountain bikers to push out the other users, who built/maintained the trails to start, so they are mountain bike only. Kind of the same thing has happened with oil companies. When the price of oil went up a few years back a lot more extraction started happening, so there is a lot more development on BLM land for that, and heck in some forest areas too. Like the mountain bikers, not all but enough to matter, some of the oil companies have tried to lock down, through locked gates and signage, public land. I have run into a fair number of "private" road signs, and the like on public roads, and in some cases the gate has been chained shut when it shouldn't be. On BLM land it is pretty much the same as forests for gun regulations, but the difference is outside of select areas there is even less use of BLM land even during hunting season.
There is a final user group that I should mention and that is the rancher. Again I am going to paint with broad strokes and focus on the ranchers that given the others a bad name. Unfortunately, it is a large enough group that it is significant. On both forests and BLM land there are what are called grazing allotments or just allotments. The rights to those allotments are held by ranchers, and traditionally the allotment goes along with a certain ranch, and has for a long time, approaching generations at this point. In theory that land is administered by whoever the local federal employee range guy is, as far as number of animals allowed graze, duration of grazing allowed, and in some cases types of animals. The point of this is that it is federal land owned by the US public (you and I so to speak) and while used for grazing it should be done in a manner that does not adversely effect the land. The fact that there is stock in an area does not in anyway shape or form legally limit the use of the area by the public. The rancher basically just has a special use permit. However, a lot of ranchers look upon the forest or blm land as THEIR LAND, and feel they have the right to keep other folks out, and do whatever the hell they want and aren't going to let any bureaucrat from Washington tell them how many and how long. This attitude can lead to clashes with both the public and either forest of blm personnel. In the case of the public there is a ton of jackasses out there and a few interactions that affect your livelyhood is enough to sour a person. Nor am not going to say that the management and oversight is not without its issues, especially when mandated through federal programs and guidelines, so I am not saying that the ranchers are completely wrong. However, some of those ranchers are hard hard guys and are not afraid to turn to intimidation, and/or outright violence (rarer now) to protect THEIR LAND from all interlopers. Locked gates and creatively hung signs are pretty normal. I am sure at this point a cause de celeb has popped into mind, because that is exactly the situation that went viral, but the guy in the wrong, in my opinion, won hearts and minds first. Honestly, there are places that I think it is advisable to carry a gun, and openly at that, but most folks are never going to get there as they are off the beaten track. Some folks will think I am paranoid, but having worked for the Forest Service, knowing LE that deals with these issues, and even some of those type ranchers myself, I disagree.
Oh yeah dope growers and mobile meth labs. Folks would be amazed at the stuff that goes on in regards to this, especially in California. If you think dope growers may be around or someone is cooking leave, quick fast, and in a hurry and report it.
Finally, there are some folks who seek out the wild places for solitude and may not be appreciative of sharing that solitude for whatever reason.
Some folks think of wild lands as being safe and peaceful, and for the most part they are, but where humans go they bring their issues with them, and luckily in the US it is recognized that we have the right to self defense. For most of the west a firearm is considered a tool like a shovel or axe. And like a shovel or axe, when you need one you should have it with you.
I hope that clears up a bit how and why folks carry guns, and why you see them photos. I will through a final caveat in, I have lived in the west my entire life and my views and experiences are shaped by that.
Co-Owner Hill People Gear
"If anything goes wrong it will be a fight to the end, if your training is good enough, survival is there; if not nature claims its foreit." - Dougal Haston