The "Law of the Land" isn't worth a **** thing, as the Indians know more than anyone. Or maybe not as much as the people who write them. There is serious money invested in this project, and if they don't clear the area by Jan 1, the contract expires, and the whole thing goes belly up. So even though the area is sovereign land, they all thought they could get away with it like they always do. The original plans had the pipeline going through Bismarck, a small city that is majority white, but the residents kicked up dust about it, so the oil company changed the route to not only run through Sioux territory, but under the **** river that is their only water source. The area has spiritual and cultural significance to them as well. The entire thing is just another travesty among many.
edit: there are lawyers who have been working on behalf of the tribe, and hopefully the illegality of police/Army action is dealt with accordingly under the law, but like I said, the law is extremely malleable when so much money is at stake.
You can also be assured that all Native American tribes are watching this very closely. The government doesn't want this to set a precedent, if the movement is successful.