​RESERVATION OF SOVEREIGNTY:

"Even if the Tribe's power to tax were derived solely from its power to exclude non-Indians from the reservation, the Tribe has the authority to impose the severance tax. Non-Indians who lawfully enter tribal lands remain subject to a tribe's power to exclude them, which power includes the lesser power to tax or place other conditions on the non-Indian's conduct or continued presence on the reservation. The Tribe's role as commercial partner with petitioners should not be confused with its role as sovereign. It is one thing to find that the Tribe has agreed to sell the right to use the land and take valuable minerals from it, and quite another to find that the Tribe has abandoned its sovereign powers simply because it has not expressly reserved them through a contract. To presume that a sovereign forever waives the right to exercise one of its powers unless it expressly reserves the right to exercise that power in a commercial agreement turns the concept of sovereignty on its head. Merrion v. Jicarilla Apache Tribe; Amoco Production Company v. Jicarilla Apache Indian Tribe, 455 U.S. 130, 131, 102 S.Ct. 894, 71 L.Ed.2d 21 (1981)