Of course, it's never that simple.
Most unpaid internships are flatly illegal. Or, they are conducted in a way that violates minimum wage laws, and a variety of labor rules. Volunteer work for a for-profit company is also very sticky, as is the provision of room and board. You don't even want to talk about stipends. In some ways, the more an employer wants to give an intern, the more legally suspect they become.
It's hard to follow the law, dammit. And I am trying.