Labor law and criminal law overlap; they aren't disjoint categories.

> Individuals are already prohibited from doing those things via criminal law. If

Yes, criminal labor laws adopted as part of broader regulation of labor practices.

> If one party is not paying the other, labor laws are irrelevant

Obviously untrue; labor laws are relevant whenever labor is being supplied, whether or not it is being paid. Indeed, the current requirement that labor being supplied be paid is part of labor law (and some aspects of it are fairly recent aspects of labor law), like the restriction on economically useful labor being provided via unpaid labor even when characterized as an internship.

Let me try putting it in a way that is (hopefully) freer from legal nitpicking: It's possible to create a system where consenting adults can engage in whatever consensual activities they wish, even when (gasp) money changes hands between them, and still not legalize objectively evil behavior like slavery, negligent homicide, etc. Further, I'd argue we don't even to make new laws to do it, though this part seems to be surprisingly controversial.

> Individuals are already prohibited from doing those things via criminal law. If

> Yes, criminal labor laws adopted as part of broader regulation of labor practices.

You're seriously saying that slavery, poisoning, and battery would be legal if not for labor laws?