The point is not that there's something unique about the expansion of the United States, but quite the other way round, that it's an instance example of a widespread aspect of human history in which what have generally been referred to as "empires" have been assembled by a process of expansion through various methods - conquest, dynastic marriage, treaty, purchase.

There's no justification for restricting the terms "empire" and "imperialism" to the relatively unusual far-flung variety, typified by the British Empire.