Counterquestion: How is what the US did "imperialism" (sans the Philippines, but we gave them independence).
Louisiana- buying excessive amounts of imperial territory off another imperialistic power to expand your territory simply because it gains you resources, territory, and prestige. Imperialism.
Texas- Americans move in until they overwhelm the people already living there, and when the people who govern the territory tell them that to continue living there, they have to follow certain rules, the Americans say 'screw that' and join America. Americans taking land they want without regards to local government=imperialism.
Mexican Cession- Then go to war with the country that owns the land you ('you' in this case meaning Polk) want, and take it in the peace settlement. Imperialism.
Oregon- I'm not going to begrudge this one too much, except for the fact that if Canada was part of the British Empire, Oregon is part of the American Empire.
Alaska- See Louisiana, above.
Hawaii- This one's so incredibly obvious it's not even worth discussing in detail.
If the British Empire had allowed American representation in Parliament, or if the Imperial Federation had formed, it would have been no less an Empire than it was previously, and only through significant, long-term connexion betwixt the countries would result in the decline of that status. In the same way, the American Empire was no less an empire merely because its scattered denizens gained the right to vote dozens of years after the land was acquired; it became less of an empire once those territories- later states- became fully integrated into the national system, politically, economically, and culturally. California was a territory gained in the conquest of an empire, and was no different in that respect than was French Guinea from France. I don't consider French Guinea an imperial possession today any more than I consider California to be one, but at the time they were acquired, both were imperial territories.
It is worth noting that I have no qualms or moral issues with terming America an empire. I don't feel disgusted at the term. It simply is what it is, and the fact of integration does not change the fact that the territories were gained as far-flung regions of an empire.