This is no different than big companies hiring unpaid interns. Sure it can kinda work in a handful of cases, but it's harmful to exploit people to perform unpaid labor.
I can recall a great many HN articles complaining about companies asking coders to work unpaid in a variety of situations. Working unpaid on learning projects in school is super! Getting experience is great! But, the reality of the situation is there are massively degenerate patent laws at work here and the solution isn't for startups to expect a contingent of students to work there buts off gratis for them.