Dave, this is not "a strictly lawful question" when politics, race, corporate interests, oil interests, environmental interests, etc. have had a play here.  If you want to talk about law, then let's talk about the Treaty of Fort Laramie, where the United States government recognized the land to belong to the Sioux. 

Then let's talk about United States v. The Sioux Nation of Indians wherein the U.S. Supreme Court ruled that the U.S. Government unjustly took Sioux land - that very same land that DAPL is slated to be built upon - that very same land that the Sioux says DAPL does not have their permission to be built on.  

Where did you get your law degree?   I got mine from William S. Richardson School of Law.