Right. I kind of disclaim the term 'wage theft' because it brings in all sorts of legal connotations like breach of contract etc. It basically says that the law is the only thing that matters.

It makes much more sense to simply reason about what is morally correct (assuming we accept capitalism and the idea that employers deserve to dictate to employees to some degree).

In the full blown Marxist world view any appropriation of surplus value is essentially 'wage theft'. Of course, if that were completely returned to workers, there would be no point in bothering with employees.

Instead, we can consider that if a worker has a wage low enough that their quality of life is really bad (e.g. complete inability to build capital as 100% of wage goes on absolute essentials), if there exists any more surplus value with which they could be paid more, it is reasonable for the employer to do so and unreasonable for them not to.

Is it 'theft' not to? Well... no. In the same way that my landlord doesn't 'steal' my labour by exploiting the fact that buying property is difficult. It's still not great though.