If someone offers a job but says he can't guarantee payment, and someone takes that job knowing the conditions and accepting the "risk" for the few minutes he spends on that "job", I don't see a problem with that. It's a free market, which means market forces should mean the "workers" pick the "jobs" with the better offer.

Those are two different things. I wouldn't offer nor take this job, but if someone else is willing to do so, why try to stop them?