Native American tribes retain their sovereignty within the United States, and if a pipeline threatens they resources, land, and well-being, it in effect threatens a measure of their sovereignty.
The indigenous peoples of America have lived here for thousands of years, but settlers took their ancestral land, and forced them into reservations as a way to provide them with their own “sovereignty.” The truth is that creating designated reservations, America was able to control its natives, to subdue their voices. They got their land, and as a result, have no reason to be angry with the settlers.
The least that the U.S. could do is respect those lands, and their sovereignties. They took everything else, all the natives want is respect.
Looking into the pipeline and the possible consequences it may have on native resources is a step to honoring those who saw their lands get taken away. It is the least the government can do for the indigenous peoples that inhabited this land before any American did.