Those guys aren't making less then minimum wage. The hard part is finding workers for one day projects for low pay.
Having worked at a work today, paid today temp place, I can attest to this. There are afew people who are legitimately in a tight spot. Most of the other workers are either ex-cons who can't get hired, people who can't pass a drug test, or hard core functional alcoholics who go on three day benders every time they get some cash.
Once again for the slow people, these are social issues, not legal issues. I do think pursuing the people hiring is a better answer. Maybe the should have to pay a fair day's wage in addition to whatever they already paid (undocumented).