Something to think about: sometimes free labor means you're populating your future industry with kids who can afford to not get paid, and this may lead to aristocracy trumping meritocracy.

It was long the practice in medicine that not only did you serve your internship for free, indeed you paid for the privilege. At some point there was a collective realization that this was contributing to, or percieved to contribute to, a good-ole-boy network instead of the meritocracy one wants of doctors. So interns, and other resident physicians, are paid.