As somebody who was homeless (not the couchsurfing type, but the sleeping under bridges type), I think it could very well be exploitative depending on how much labor is involved and what profit is made from them. I'm not sure what you think minimum wage is for, but I think that keeping poverty from becoming a profit center is one of those things.
If people weren't available to work for less than minimum wage "willingly" there wouldn't be a need for it.
If they retain all of the profit, then I'm all for it.
Have you lost your bet?
edit: what if they were being paid $20 a day for being tables, or coatracks? Would it be exploitative yet?