Even though it is an unpaid internship, if they are doing duties which someone can be paid for...(ie, an unpaid intern in an office might answer phone calls....even though they are not being paid, a US citizen COULD be paid to do this, therefore this is not allowed without the correct work authorization), then no, they can not legally be an unpaid intern.
It depends on the role itself. If there are other interns doing the same work unpaid, then it is likely to be kosher.