The very idea that it could be forbidden to learn a language seemed nonsensical to me at first, although upon reflection, it must have been a good way to keep foreign ears from prying too much (I believe China's ban on foreigners learning Chinese was more about counterintelligence and less about wanting Chinese speakers to have some deep, authentic connection with the culture).
I hate stuff like that hostility between Hopi and Navajo people though. If anything, learning a neighbor's language tends to bring people together!
I tend to agree, but I also respect the wishes of the various groups. They've undergone experiences at the hands of colonizers that I can't begin to imagine, and if they want to try to preserve something of their culture so that it stays in their culture, and not just let any outsider with a passing interest learn it, I support them in that. Though I do wish more were like the Cherokee Nation, which offers free online courses for anyone across various levels of Cherokee, also integrating culture and such.
It is actually illegal, as in there is a tribal resolution, for me to provide any demographic information to any entity without tribal permission.
The worst part is those folks who say they are Native American from a particular tribe and are NOT on that tribes enrollment rolls. They are always fun to deal with.
For a lot of Native Americans/First Nations peoples, it's actually a highly contentious issue. Some of them don't want outsiders, with no connection to the culture or the group, coming in and learning it just to prove they can, especially when children of the group haven't had the opportunity to learn it because of colonialism and other, often horrible, things.
The Hopi are known for this, and actually decided to forego teaching the language at a school because they couldn't guarantee Navajo students wouldn't sign up for it.
So, really, there's a lot more involved than it just being an 'endangered language' that everyone should have the right to learn and that they should want everyone to learn. There's a lot of ethnolinguistic and postcolonial issues involved as well.