> Why would it be?
This isn't the culture in the US, but in some parts of the world, the default assumption is that land is accessible unless it's fenced or otherwise marked. There are some politeness norms like not walking right up to someone's house in the woods, but generally in i.e. Norway, if it's outside, you're probably allowed to be there.
The culture in the US seems to come from capitalism being such a part of our culture. Every resource, including land, is viewed as a commodity. Even many of our publicly-owned land which is ostensibly intended for public use, such as state and national parks, has entry fees.
I don't think there is any doubt that these military bases were marked.
> > And if there's no fence, isn't it just public land that it's OK to walk on?
> Why would it be?
Although land does need to be marked with "No Trespassing" signs in order to gain certain rights under the law.
It costs money to maintain public lands like national parks. The neanderthals who don't understand Leave No Trace principles cause massive amounts of damage that needs to be prevented, policed, and repaired. Chores like trail maintenance, habitat restoration, wildlife studies, trash removal, toilet upkeep, etc. etc, all need to be done to keep the parks as close as possible to wild, so that the next generation can enjoy it. Entry fees fund the conservation of these public lands.
It costs money to maintain public land, but that's not any less true in the other countries I'm talking about. The difference is that that cost is more likely to come from national budgets, rather than from fees, which is better in a lot of ways: it's more transparent, and is a progressive rather than regressive tax, is better for tourism, etc.
Also note that a lot of the non-fenced land in i.e. Norway isn't public land, it's private land, and it's still accessible to the public due to cultural norms. This may seem weird that there's a social expectation that people will open their private property to public use, but the flip side is that there's little cultural opposition to government helping maintain property. Many hiking shelters, for example, are on private property, but built and maintained by the government.
In the western US, you will find several remote military bases that include ground and air live-fire ranges that are un-fenced. There are usually signs though. (Some) military bases are very large, making it impractical to maintain complete fencing around them.
>And if there's no fence, isn't it just public land that it's OK to walk on?
Why would it be?