imperialism describes a relationship between a country and the rest of the world. the us is imperialist because it subjugates and loots other countries. how it treats its own people on its own territory has nothing to do with whether it is imperialist. iran is not imperialist because it does not subjugate and loot other countries. how it treats its own people on its own territory has nothing to do with whether it is imperialist.
the us is impirialist because there is a us empire. iran is not imperialist because there is no iranian empire.