The facts on the ground are that, dissuasion or not, these economies will never go away ("oldest profession", after all). Full legalisation would allow for complete oversight of such murky sectors, which would ensure everyone's safety better than the current arrangements, at all levels. For example, in many countries there is no way for prostitutes to legally ensure their own security, because any sort of professional relationship with a sex-worker is illegal (even if the business itself is not); at that point, whether they have recourse post-abuse or not, is basically irrelevant.
The legal situation around cannabis and prostitution is just a shitshow in most places, driven as it is by outdated sensibilities which have ossified through short-term political calculation. There is no point trying to find in it a logic that is simply not there.
Full-on legalisation is not universally desirable, though — there's always heroine, cocaine, crack, meth... — and the asymmetric legality thing is still a very useful tool in those circumstances.
There are some truly horrible substances, but with current approach heroin and weed are sold by the same guy.
This seems weird, but is actually a perfectly reasonable legal regime that's all about power dynamics and enabling the authorities to approach people. Drug users and prostitutes are in much more vulnerable positions than drug dealers and johns respectively.
If you make selling drugs a crime, but not buying/owning those same drugs, now the police can approach addicts much more easily, and get them help (this is the much-lauded Portuguese model).
The same logic applies in reverse to prostitution: if selling sexual favours is legal, but buying is illegal, prostitutes are enabled to report abusive johns to the police, and authorities can approach them much more easily to try and get them off that life too.