This will be an unpopular opinion, but I am feeling chatty today.
As long as safety + working conditions are not cruel and unusual, and someone is willing to work for $0, why are you trying to stop (or hide) that fact?
Someone willing to work for less (or for free) is offering a bargain to incentivize giving him/her the business or the benefit of experience instead of more $. You restricting them from doing that doesn't magically solve their problem of not having a job, you realize?
We have tons of people who are locked out of well paying jobs because they can't get on the job experience or even get in the door at the wages being required (and experience demanded by those wages). Young people, people with less education. Minorities. Did you know that? It actually causes some people's heads to explode, that the good intentions of a policy actually might hurt the people it's designed (in a very coarse way) to benefit.
This is the classic conundrum of the minimum wage. You may not realize it but you are implicitly choosing some people to benefit over others.
It's great for people who can get the job. It does nothing for those who cannot get employed, who for many industries are more numerous. It's in fact a very discriminatory regulation in outcome (although of course not in intent -- no one intends that obviously!). But intentions, as they say, are heavily overrated. Look to outcomes.
Maybe the person doing unpaid work is someone who has been overlooked by "the establishment" and is trying to break into the industry. Maybe it's a 2nd job to support the family and doesn't have to be a full wage earning job. You want to foreclose people from those possibilities and say that only full professional (say, union members) should be able to get that job?
You are not going to escape the fundamental fact that there are people who are out there wanting to work, but who cannot command the pay rate dictated. And that we have an excess of labor in many pockets in this country (really, we do!). You just hide the problem by saying that it's unjust to pay so low a wage and not allow people to do it. The people and their lives are not going away just because you mandate not to see them.