(Some) US departments seek to maintain a critical mass of US students. (I expect that) such departments accept a critical mass of US students - the best that they can get - and then accept some number of international students to fill up their ranks - the best that they can get. Since international students generally score higher on standardized tests such as the GRE general/subject test (for all sorts of reasons), this leads to the accepted international students having higher test scores than the accepted US students.
A more general point, I believe, is that international students are often 'unknown quantities'. While a US student might have recommendation letters, research experience, etc. that gives an admissions committee a well-rounded perspective on them and could potentially make up for less-than-stellar test scores, this is not always possible for international students. So in some sense an international applicant has to have exceptional test scores to make up for being an unknown quantity in other respects. I believe this partly because international students who have been undergraduates in the US seem to fall into the `domestic students' box more so than the 'international students' box.