"Almost all" isn't really accurate, as a substantial chunk of the US is actually federally-held public land (https://en.wikipedia.org/wiki/File:US_federal_land.

"Almost all" isn't really accurate, as a substantial chunk of the US is actually federally-held public land (https://en.wikipedia.org/wiki/File:US_federal_land.agencies.svg https://en.wikipedia.org/wiki/File:US_federal_land.agencies....). Most of this, such as basically everything organized under the Bureau of Land Management, is available for public use with no permits or other restrictions beyond what amounts to 'don't damage the land' rules. That's before accounting for state-held public land, of which there's also a significant amount.