It isn't. The land in question is claimed by the tribe, but the government doesn't recognize the claim. They didn't win on the land ownership claim. They won on the potential harm claim. That claim is also ridiculous BTW since the government is already in the process of moving the intake for the reservation water supply far downstream due to the water being too shallow where it is now.
Tom Woods recently did a whole episode of his podcast on this topic.
Ep. 793 What Should Libertarians Think of the Dakota Access Pipeline?
You meant to say up stream as the Cannonball flows South to North until it hits the Missouri.