Im British so my knowledge of it isnt spectacular, but it seems to me that this policy was similar to European expansion in places such as Africa (as in "Natives are inferior to us, so we must take their land and 'civilise' them"), and yet it is not held in the same level of disdain. Am i completely wrong about this or what?
It was a form of colonialism. Colonialism is defined as "the policy or practice of acquiring full or partial political control over another country, occupying it with settlers, and exploiting it economically."
The USA took its Western territories by war or purchase and flooded them with settlers. In many ways, the colonialism of the American West was more pronounced than European colonialism in Africa. This is because in the American West the natives were displaced and killed, and forced onto reservations. However, in Africa, Europeans existed only in small communities (South Africa excepted) and governed through existing ruling elites.