Well as I said, there is no thumb rule that you can get unpaid or voluntary internship/work in US (including universities)
It depends on the student - how well he/she tries and it depends on the employer if they want free help or no.
But this thing for sure that not many places welcome voluntary because of BCIS tightening the noose on univs, they would rather pay, but again its case by case. It depends on how hard you try.