This was the old model of the United States when we had a frontier. The frontier, land owned by the natives, was deemed available for the violent taking by the settler-colonials. Whenever sufficient ferment began to develop in the US proper, they would push for a new wave of expansionism which would act as a pressure release valve.
I don't understand unincorporated land, but clearly someone "owns" it or the government wants it for some reason and the former era of free expansion and manifest destiny is over. Now we must reckon with the damage and reparations owed to Native Americans.