Its not a crime in some parts of Nevada.Here's a better question.
Why is prostitution a crime to start with?
I meet a woman in a bar, and after a few drinks, we go home and have sex, and then never see each other again. Not a crime.
I meet a woman in a bar, and after a few drinks, I pay her $300 to go home and have sex, and we never see each other again. Why is the second one a crime and the first one is not?