Grasslands like Head-Smashed-In were maintained to attract buffalo, and timed to attract them in prime harvest season.
Burning was done to improve productivity of berry patches, etc.
It just doesn't match the European idea of agriculture / settlements.
Prior to the horse they were not nomadic, the prairies don't support a nomadic way of life without some animal you can ride, and the prairies didn't have one that can be tamed, so it is clear they were not nomads. The natives didn't have access to the types of building materials that would result in permanent structures, which means there is limited evidence for archeologists to work with, but there is enough about what life was like before the horse to know it wasn't nomadic.
Once the horse came many became nomadic as the horse allowed a nomadic lifestyle that wasn't possible. It wasn't long after the horse those that Europeans wiped the natives out (small pox and the like also wipes out a lot of natives and their culture). There is reason to believe that if the Europeans had brought the horse and then left the nomadic way of life was on the verge of causing a collapse of the Bison population, which would have force major changes (most like a return to farming settlements, but massive starvation followed by some form of culture population control - or something else on these lines that you can think up)
The switch to a nomadic way of life (which was directly caused by the arrival of Europeans) cause the loss of knowledge about the older ways of life. We have to make a lot of guesses, but if you know what to look for some things become clear.
Note that we talk about natives as if they are one culture, but that is not true. There were many different cultures with different ways of life. Some adopted nomadic ways of life more than others. The horse allowed the nomads to out compete those who tried to retain the old ways, so those who didn't adopt the horse either died out (starved, assimilated) or had to move to areas that horse didn't do as well in.