If something is legal then workers have protection, can form unions and have rights. It's more difficult in a legal environment to have people who are coerced or forced to work.

If prostitution is illegal then it will be run by people who do not obey the law and will happily traffic people and have people people working illegally or forcibly.

http://humantraffickingsgp.weebly.com/amsterdams-red-light-d...