If my neighbour chooses to work as a prostitute in their apartment, or rent their apartment to someone who happens to be a prostitute - that's not nearly the end of the world, you know, they're people too, just with a comparably hard/unpleasant profession.
An active neighbour can easily be bringing some stranger they picked up at a bar twice each night; if that stranger pays afterwards then that doesn't change anything for me.
In this case it's a prostitution ring, which isn't really the same as a long term neighbor who happens to sleep around a lot. I think someone who calls an apartment home rather then stopping there once and never coming back will be more respectful.
You seem to be saying stable prostitutes or brothels are OK even though they are also illegal in those buildings. I agree with the parent that a promiscuous person is effectively a prostitute with the distinction being only an arbitrary legal one. As you say though, the temporary nature of their stay is probably the real problem.
An active neighbour can easily be bringing some stranger they picked up at a bar twice each night; if that stranger pays afterwards then that doesn't change anything for me.