They did commit a 'required act', theys solicited a minor for sex by internet, phone, or person. They know that the person is underage, and they agree to meet up with them to have sex. Now, if they stopped there they'd be fine, but when they actually GO to the persons house, thinking there is no adults just the underage kid and bring condoms/beer/weed whatever, they have probable cause to be arrested. The group who conducts this, "perverted justice" they're called, work hand-in-hand with the police so they're evidence is legit.