While it is true that they have their own tribal police, they exist for simple criminal behavior. Federal felons, insurrection, etc. would all be subject to Federal authority if they became obvious.
My point, is that entire system is archaic and outdated, and certainly not in the best interest of either the United States as a whole, or the Indians living there themselves as eveidenced by any number of clear factors.
IMHO, as an individual who lives out in the west and sees these conditions and hears of these radicals fairly regularly...that the entire system needs to be done away with and the people engrafted into the nation.
But that is just my opinion.
I will bet you this, if these radicals made any overt moves to try and establish in fact and reality twhat they talk about (ie. began blocking off interstate highways at their "border"), Not only would you see the US Federal government immediatly squash it...you would see a civil war amongst the Indians themselves.