No, actually I think it means that no crime has been committed. It isn't illegal to pay for sex, but nor is it legal: it's treated as if there was no payment. It's the contract that is unenforceable: no contract, no obligation, no crime.
In effect, a WG has a special exemption that allows her to legally rip off her clients.
It came up awhile back, when a now-blacklisted WG (Scarlette XXX) ripped off a client who had paid her in advance.