Fiddling While Rome Burns
Carthago Delenda Est
"I used to roll the dice; see the fear in my enemies' eyes... listen as the crowd would sing, 'now the old king is dead, Long Live the King.'.."
It is legal. Film it and call the prostitute an actress, bam legal.
The supply in prostitution is fundamentally different. Women willing to sell sex don't grow on trees. You can't just build another prostitute factory. If the demand for sex (men willing to buy) is larger than the supply for it (women willing to sell), there will be a black market for sex.
Let me rephrase the question. Why would sex trafficking rings give any information to regulators regarding their operation? How can the regulators inspect the operation if the whereabouts are kept secret? Sex traffickers hide their operations from law enforcement. Why wouldn't they hide their operations from regulators?
But it doesn't actually matter why. It's a known fact that sex trafficking still flourishes in nations where prostitution is legal, despite your insisting it magically goes away. In fact, the evidence shows that sex trafficking actually increases as a result of legalization.
Human Trafficking Persists Despite Legality of Prostitution in Germany - SPIEGEL ONLINE
Does Legalized Prostitution Increase Human Trafficking? by Seo-Young Cho, Axel Dreher, Eric Neumayer :: SSRN