I got into grad school because I worked for free. To get into grad school in science, you need to have professors that are interested in supporting you (at least partially supporting you, as once you are in you can teach to get department funds). As someone who had been out of academia for a decade, I was having trouble finding any professors who would take the risk of supporting me and I couldn't get accepted to school. Then a friend of mine who was a recent PhD grad told me that I should just offer to work for free. After a few months of working for free, the professor I volunteered for, as well as few others, were willing to say they would support me. Lo and behold, I was accepted when I applied again. I think in many cases, interns should be paid, but I don't think a blanket law requiring them to be paid makes sense. I would never have been able to get into the school I wanted without volunteering my time first. The professor I worked for simply didn't have any money to pay me, though I'm sure now that he would have it he could have. It's just a conjecture, but I bet there are other situations where volunteering first gets your foot in the door where there simply would be no other option to open that door.