You implied that employers always have the power in the relationship. I'm pointing out that it can go either way. Obviously people working for free must be in a weak negotiating position.
Even still, nobody's forcing them at gunpoint. Presumably they chose a an unpaid graphic design internship because they value the experience. They could have chosen to work elsewhere for pay. As long as they are not being forced, are not being deceived, and are choosing according to their own priorities, it's their decision.