I really like this angle. I'm especially impressed with the idea of keeping the prayers in Lakota rather than English. It's like how the Catholics used to stick with Latin. Language influences thought; it's very powerful; and that's an astute ruling.
People can come in from outside, but they have to do things the Lakota way. If you do the work, then you're sharing, not stealing. If you just want to grab stuff and run off with it, that is misappropriation. You have to learn how to do things the right way. It's the difference between a McDonald's burger and a buffalo steak. You have to work harder to get the steak, but it tastes better and is better for you.
This is one of the best examples I've seen recently of appropriate cultural boundaries. Don't shut people out, don't let them rob or abuse you, manage the interaction in mature rational ways. Aho!