The difference between the relation of Kansas with Luxemberg and the relation of Kansas with an Indian tribe within its borders is that the United States has never conquered Luxemberg. Back in the 1820s Chief Justice Marshal ruled, in Johnson v. M'Intosh, 21 US 543, 587-89 (1823) that the United States had the complete power, by right of conquest, to extinguish Native American sovereignty. That power was also exclusive; the states could not regulate Native Americans except insofar as authorized by the US to do so:
The United States, then, have unequivocally acceded to that great and broad rule by which its civilized inhabitants now hold this country. They hold, and assert in themselves, the title by which it was acquired. They maintain, as all others have maintained, that discovery gave an exclusive right to extinguish the Indian title of occupancy, either by purchase or by conquest; and gave also a right to such a degree of sovereignty, as the circumstances of the people would allow them to exercise.
The power now possessed by the government of the United States to grant lands, resided, while we were colonies, in the crown, or its grantees. The validity of the titles given by either has never been questioned in our Courts. It has been exercised uniformly over territory in possession of the Indians. The existence of this power must negative the existence of any right which may conflict with, and control it. An absolute title to lands cannot exist, at the same time, in different persons, or in different governments. An absolute, must be an exclusive title, or at least a title which excludes all others not compatible with it. All our institutions recognise the absolute title of the crown, subject only to the Indian right of occupancy, and recognise the absolute title of the crown to extinguish that right. This is incompatible with an absolute and complete title in the Indians.
We will not enter into the controversy, whether agriculturists, merchants, and manufacturers, have a right, on abstract principles, to expel hunters from the territory they possess, or to contract their limits. Conquest gives a title which the Courts of the conqueror cannot deny, whatever the private and speculative opinions of individuals may be, respecting the original justice of the claim which has been successfully asserted. . . . These claims have been maintained and established as far west as the river Mississippi, by the sword. The title to a vast portion of the lands we now hold, originates in them. It is not for the Courts of this country to question the validity of this title, or to sustain one which is incompatible with it.
Consistent with that general principle and other Supreme Court precedents, Bryan held that the states could not tax Native American property without the consent of Congress, and that P.L. 280 had not given that consent. Congress could give that consent. It could end tribal sovereignty tomorrow if it chose. Bryan refers to several "termination acts" in which Congress did just that. It could also prohibit gaming on Native American land, or even place that land under state jurisdiction for civil and tax purposes. To the extent it has declined to do so, that is a matter of forbearance, based on the conquerors' sense of what is just.