Wait a sec, wasn't the majority of that land in the western states claimed by New Spain and then Mexico? How is the maker of this map qualifying "land of native nations"?
Then it's fine to fasten a noose around the natives' neck and very gradually tighten it?
What else could you be indicating by posting about this in this context? It's true that natives were nomadic people moving through the land, but how does that make it right for europeans to come in and claim land in a permanent fashion from across the globe?
The lands you are probably referring was the Mexican Cession (most of the US western lands now). That cession happened after the Mexican war that ended in the treaty of Guadalupe-Hidalgo signed in 1848. So the map mostly accurately reflects that as US territory in 1850.