<p>I think it has more to do with the funding opportunities for internationals…NSF and NIH grants are limited only to the US citizens. most of the internationals who apply to graduate programs do not have their own funding and the university especially the department has to pay for their costs. Because of the fact that almost all of the science graduate programs are funded by NSF/NIH grants, they have to limit their international students number to a few.
I think this information is posted in the most of the programs’ admissions webpage under international student admissions.</p>