Isn't that kind of like saying "it's fine to have hit-for-hire ads in newspaper classifieds[1], because if hit-for-hire is really a problem they can just go after the hitmen/clients?". I think most people wouldn't tolerate criminal activity being advertised in the open, even though they know forcing such activity to go underground wouldn't prevent the criminal activity from happening.

[1] ignoring how you'd anonymously place/respond to the ads