(or so the argument goes for unpaid internships)
It suddenly becomes a lot less palatable when it's your livelihood on the line.
Because I have skills that produce value to them that is at least what they pay me. Interns often are so lacking in skills and work habits that the net value of their production is zero, or less when you consider the time investment of others that have to supervise them. Why is that so hard for you to grasp? Go ahead and ban unpaid internships. Who exactly would gain from this? Certainly not the intern. The company wouldn't care. They can just cancel the program that will now cost them more than it's worth.
Isn't it a better world when busy bodies, with childish and disgustingly obtuse ideas of economics and what's fair, just mind their own business and let people enter into relationships that suit them?
It’s a question of Return On Investment (ROI).
For an established professional, the company gets positive ROI because of the quality, relevance, and volume of work produced by the individual. Therefore, the ROI merits paying the individual commensurate with his or her performance.
For an intern, the company trains the intern while receiving very little (if any) usable work product in the profession being interned for. Therefore, the amount of investment that is merited by the return is much, much lower - and in some cases is (monetarily) nothing.