Foreign students pay higher tuition than local in-state students.
Which means the school make a bigger profit from each of them.
That's why the schools want them.

Nonresident Tuition Fees

In addition to the tuition fees in the applicable charts above, out-of-state and international students must pay non-resident tuition fees of $372 per unit or fractional unit.
For information on Fall 2014 fees for Non-Resident and International students, please visit the Bursar's Office website at:
http://www.sfsu.edu/~bursar/fall2014..._schedule.html