Is US experience usually required for TT jobs?
This is a bizarre requirement that I have never seen and sounds unwise to me, except maybe for a position requiring specific U.S.-centric expertise, e.g. a search for a law school faculty member specializing in U.S. constitutional law (even then, requiring a U.S. PhD/postdoc sounds to me like the wrong way to ensure that the candidate possesses the relevant expertise). And no, I do not see why it should matter that this is an Assistant Professor position.

Based on my experience with bizarre/illogical rules at organizations, I'm guessing that what happened is that the department that is requiring this at one time had a really bad experience hiring someone without a U.S. background, and after that someone there decided to make sure that that particular mistake would not be repeated, no matter how many good opportunities that would mean missing out on.

Note that while I think this is a silly requirement, at many U.S. universities a tenure-track candidate who has no U.S. teaching experience could reasonably be considered by some people to be less competitive than another candidate who possesses such experience, all other things being equal. But this judgment call does not require a special rule, since being well qualified to teach to the standards of the university is already a requirement of the job.