> Indian tribal casinos are only legal because they are in a separate country as far as the state is concerned. (they don't bother, but each reservation has a good case to join the UN if they wanted to)

A case for it, maybe, but I think the way you've worded this is a bit of an exaggeration. As you said, they're separate as far as the state is concerned, in some respects (even that's somewhat case-by-case and debatable). Comparatively, it's much more generally recognized that federal law does apply to tribes. The US calls tribes "domestic dependent nations," which is short of independent sovereignty, and the UN would generally only admit members who are recognized as independent and sovereign. Native American tribes are "separate countries" like states are separate countries-- in principle, kind of, but in practice, not really anymore. Individual states can work with UN bodies on projects (like California recently joining the UN health network), but they can't be admitted as members.

All of that said, gambling is, of course, a very well-known thing that's allowed to happen on tribal lands.