[Sangermaine]

This is quite a complex question, but the basic answer is that under US law native tribes are "domestic dependent nations". This means that they possess sovereignty as any separate nation would (which flows from Constitutional recognition of their pre-existing status), but it is a limited sort of sovereignty. Congress can limit that sovereignty through legislation and the federal government can make treaties with them. However, unless a treaty or federal statute removes a power, the tribe is assumed to possess it.

In practical terms, addressing your question, if tribes keep trying to sell their sovereignty like this Congress can pass a law removing their sovereign immunity with respect to patents. And, in fact, moves have already started in that direction:

https://www.reuters.com/article/us-allergan-patents-congress...

The tribes would probably be wise to knock this behavior off as it's almost certainly going to result in pieces of their sovereignty being carved away, because everyone seems to agree that this sort of scheme is an absurd abuse.