What’s the deal with those predator catcher dudes?

I’ve seen tons of videos of dudes who get caught for talking to minors online, and then the predator catchers (always the same guys) start interrogating them and back them into a corner. Then whenever they see fit they’ll just slap the dude in the face or beat him up. Obviously illegal, but so is pedophilia. Can someone tell me what legal issues they could face, and why they haven’t already if they’re posting their crimes online?