You've missed the point. The original premise from patio11 was that legalizing prostitution doesn't automatically make it all safe, aboveboard, and free from crime. Then he used Japan as an example... except that prostitution isn't legal there. Regardless of how it's treated by the police, the point is that it's not legal, and that skews how the illegal operations work.

(Regardless, my feeling is that Japan's attitude toward sexuality is so different from that of Europe/the Americas that it's a little hard to make good comparisons anyway.)

The Amsterdam argument is interesting and worthy of further discussion, at least.