"Saying "you're free to walk away" is not a moral argument; if the only way a young person can find work is through doing voluntary servitude then they are stuck in that position until they do so."

I disagree. My unpaid jobs have always been much more accommodating when I leave - they realize they're not paying me so they don't give me a hard time at all. It's more of an attitude of, "We knew this day would come. Good luck, and if you need references or anything feel free to ask." My paid jobs have generally been a bit more stubborn, like leaving insulted them.

Also, many unpaid internships give at least enough experience to have something to talk about in a job interview. I know my unpaid work was critical in landing a paid internship in January. While some people may take advantage of students and the poor work environment, you can't make a blanket statement and say every unpaid internship is unequivocally bad.