The rhetorical phrase "democratic empire" is patently silly. Internally, it's an ideological oxymoron. Externally, Germany and Japan - upon whom we forced democracy - are hardly subject dominions.
I've never understood that criticism either. The US is a weird sort of empire -- one that spends hundreds of billions of dollars of its own money to help other nations while not demanding any financial favors in return. One can, of course, disagree with the usefulness of such a policy but repeatedly calling the US an empire for engaging in this behavior is getting tired.