I think the difference is people going out of their way to work for free at a company they enjoy and wouldn't get a chance to work at normally, vs. the expectation that you must work for free, like the traditional interns for Lawyers.
You're right, in the latter instance, there is such a large supply of interns working for free that it becomes the expected route of entry into these firms and creates a big market distortion. You could argue these tend to be for high paying jobs requiring special skill-set, and thus could view 'complusory' internship as an extended 3-month interview.
But if you go and work at a start-up for free, you're there because you want to, not because it's expected.