Because Puerto Rico, Guam, US Samoa, etc.., don't count even as they continue to this day?
Oh, and something collectively known as Indian Reservations?
The post Revolution US spread into land West of the Missippi?
US/Mexican War?
Liberia?
Meddling in countries throughout the world?
150,000 Guatemalans DIED for the sake of Chiquita Bananas, what do you call that?
And yes, what the Union did to the the Confederacy & the post-War South WAS imperialistic, and no, it wasn't because of human rights.