Yes and just like people who harbour people to work for them without paying taxes can get raids by a task force to search for them...Although, if you don't mind me asking: who says they DO register everyone who's there and not hide some people?
Example:
I take a car and drive it around a bit without license, who will know ? No one unless someone stops me and asks for my drivers license.
Legalizing and controlling by setting up task forces and conduct regular searches/raids go hand in hand.
I didnt even knew Amsterdam had such an appeal, to me its just another capital in another country. There are enough places in this world where you can do whatever you want and get away with it by using the correct leverage (read bribe), the only difference is that in the Netherlands they are using common sense.However, I do believe that much of Amesterdam's appeal is a gimmick. There are not many places in the world that offer similar laws and because of that, they get a lot of tourism. In truth, I'm sure Amsterdam would hate it if the US legalized prostitution as they would probably take a major hit on their own tourism.
You cannot wipe out corruption , underground activities etc if the soil where it can grow is enough potent to sustain it. And clearing that ground of its potency is exactly what they do over there.