In general terms I agree that bans promote a black market that can be more detrimental than without a ban (but not necessarily). But you could say the same thing about prostitution, child sex trafficking, drugs ... etc. Banning those things also encourages a black market to flourish because the commodity in question become harder to obtain, and therefore more valuable. So I think the author needs to re-work his logic a little. Depends on what you mean by "ban." There's punishment, of whatever level. And then there's "we'll hunt you down and kill you." Adults indulging in mutually agreed upon but proscribed behaviour like prostitution, dangerous drugs, etc, is the former thing.
Forced or child prostitution, slavery, etc., is the latter thing, or should be.