Its a bit like gun or drug laws. If its legal or tolerated by the law and openly available then the trade will attract a lot of customers and be relatively large.
If the trade is outlawed and policed as such then the trade will still exist, but to a much smaller extent. At least if the trade in underage male prostitutes was policed the correct people would be getting nabbed. As it stands now the cops simply round up the victims for a media show while the organizers act with impunity. Of course we all know that it is not in the interests of Thai police to enforce Thai law because it would mean a loss of extortion money.