[davmre]

Here's one reason: a lot of state schools (Berkeley, for example) charge a lower tuition rate for in-state residents. Americans can gain state residency after a year or so of living there in a PhD program, but international students are always nonresidents. So supporting an international student is significantly more expensive than supporting a domestic student from the perspective of whatever research grant or departmental fund is paying your tuition.

[hudibras]

Oh, that makes sense. Thanks.