I don't think it's derailment.
I really believe that only violent offenders should be locked up - In this case, she should be locked up, but the sentencing is a bit weird. There is so much we don't know and that makes the precedent this judge has set a little bit scary. There is no proof for how that fight started, we haven't been told about that violent girl's previous offenses, the only proof of motivation is the attack itself (as though it leaped into life on the camera without ever being born), and the word of those directly involved (I sincerely doubt she got up and told the judge "I am filled with hate, and she was my outlet...").
I could never, ever be a judge. I could never, ever be a lawyer or police officer. I have seen too much, lived too much of the ghetto lifestyle. Tekla is right, many of these people feel as though they are going to go to prison at some point anyway, and many are jaded by dealing with the system their entire lives, so they might as well be all in when that wall breaks down. Harsher sentencing is actually doing them a favor. Honestly, life out on the street makes you believe prison is actually an option, and sometimes a better place; a means of escape and provision if you will. This girl who is going to prison may actually end up a repeat offender who lives in prison forever - and it can probably be construed as an improvement to her life direction, a place she MAY end up committing more serious future offenses to get back into.