Prostitution is illegal, right?  There's public policy behind it, including victimization and other issues which are better discussed elsewhere.  So we've met the element that "criminal law is to prosecute and prevent crime".

I'm not following the hidden logic of prostitution-sting being "another species of gangster".  Gangsters don't prosecute crime, they commit them.  As we've already established, prostitution is a crime.  

If you want to partake in the lovely pleasure of prostitutes, go someplace where it's legal, like Nevada (outside Las Vegas), or Amsterdam.