How is the Playboy Mansion not charged with prostitution?

Everybody knows about it, so it's not like it's secret. It has magazines, TV shows, and much, much more. What I don't undestand, though, is how it's not considered prostitution and how Hugh Heffener's not arrested or fined. He pays the girls to stay there, and men do pay to have with them. I just don't get it. Can anyone explain?