It's unacceptable to lie to potential interns about what they can expect, but if someone thinks they're getting some acceptable non-monetary value for their time and effort and wants to so the work, I don't see a moral problem.

Whether having unpaid people help with your work is good or bad is something else. They're going to cost you your own time and effort, so you need to know if you're shooting yourself in the foot by skimping on payroll.