I don't know about the professors, but one of the reasons that most of the TAs are foreign is because that is one of the few jobs they are able to have in this country with a student VISA.
When I was a TA at Purdue, the kids used to tell me that I was their favorite TA because I was the only one they had who spoke English natively. I had those little suck-ups trained to bring me food during labs too.