Exploiting is a tricky word, the argument that they have used in the past is that they more or less employ them as a charity and if they had to pay normal minimum wages would not.

I don't know the specifics of what people were making, but when I was working at Burger King in college we had a disabled person who did approximately nothing, and had a caretaker in the lobby waiting for the shift to end the whole time. But it was good for him to get out and interact with people.