If the unpaid labor is FORCED or COERCED, you could have an argument there. But when the unpaid labor is voluntarily given, then it bears no resemblance to slavery. And if you have multiple applicants clamoring for the opportunity to work in one of those "worse than slavery" internships, then what does that say about your argument?
How is an internship ‘voluntary’ if it the *ONLY* way into a particular career, regardless of company or employer?