So, because they paid him to have sex with these girls, are they not guilty of prostitution? And because they were providing the entertainment as well, were they not pimps? And since they used the governments money to solicit sex for they're "informant" and then paid him with our money, didn't they illegally use our tax dollars?

Yea, it was the girl trying to make a few bucks that is a criminal here. The victim was the guy so offended about being asked that he went back four times to actually have sex with them.