The Indians didn't just spring up out of the North American soil, they came here from somewhere else just like the Europeans. Indians were not the wonderful wise stewards of the land history is whitewashed with. They roved in part because they used up what was around them, they also killed Buffalo en mass by running them over cliffs when they could, they killed other tribes and enslaved them over territory. Were whites really the first slave owners? I think not. 
You usually find this sort of historical revision only in dictatorships and communist countries, interesting how it seems it's always democrats that want to revise history in America.
Yep, everything from torture to ritual cannibalism are ignored. My wife's people were known for being able to keep a man alive for a week is seriously degraded (and frankly, unsurvivable) condition.
The biggest reason the plains people tended to be migratory was that game became sparse and campsites became fouled after a bit, and moving was healthier. A friend did find an area where there were plants (edible tubers, fruit, berries) all grown in distinct patches, so that no matter when someone came through from mid spring to late summer and first frost, there would be something to eat. While he only found one, I would wager more of these 'gardens' were scattered across the landscape along paths of movement. What hasn't fallen to the plow or development otherwise might make an interesting botany/archaeology thesis, if those areas could be found.
