Well, think of it like this: these teenagers take pleasure harming defenseless animals.
They like to torture them psychologically and physically, precisely because they are defenseless.
Well, these animals are just big animals: human.
It means: they find it fun so they actually enjoy harming humans.
This is precisely the reason for bullying.
Punishing these behaviors early, and you might actually stop this pleasure-loop and send a signal to all people around that it is a not a good idea. In addition, you may prevent escalation to worse crimes. Once you do a crime, then crime+1 is maybe ok. If crime+1 is maybe ok, then crime+2, etc.
Pithy version: Hitting kids to show them that hitting other kids is wrong?
Less pithy version: The message you send by beating kids, is that violence is wrong unless you're big and strong enough and have enough authority that nobody can stop you. This is not a good way to get kids to be less violent, it just teaches them to be more calculated in their violence.
another take: some people are emotionally “dumb” and have a really hard time feeling empathy for others. This is just another way to force the bully to be in someone else’s shoes…literally.
Most bullies are responding to a poor home life where they are bullied and beaten.