Because it's part of law. You're supposed to keep context in check. If a kid comes up and slaps you repeatedly on the face, you can't just whip out a crow bar and smash his face in. Look it up.
It sort of taints the story that the bigger kid got 4 days, but he still ed him up nonetheless beyond what was necessary to subdue that tiny little . I agree with SFIE, no sympathy for that , but there are laws that state you cannot just retaliate in any type of defense that you want.
If a woman starts hitting me, it would be unjust under the law for me to body slam her into the ground and bash her face in until it is entirely ed up, and then curb stomp her. Just the way it is, even if the person deserves most of it.