As long as there are people willing to work for nothing or to pay for the privilege of working for free, of course some "employers" will step up and take advantage of them.
It has a cascading effect too, for the true "paid employees" of those venues, because if they truly do mentor and train their unpaid "helpers", sooner or later their own worth may become an issue. Why pay higher wages and give benefits to someone who has a younger/cheaper/more eager protégé just itching to step into that spot?
Mentoring/interning works when there is an expanding job market, and companies want to get the best & brightest, already trained & ready to go, as jobs open up. When they are just using these young people as free labor in a tight job market, it's a nasty thing to do. It "tells" paid-workers "You better not ask for vacation time, raises, benefits, or you might be out of a job", and it misleads young eager people into thinking that if they work hard enough/long enough, they will soon be a "real" employee.