While I agree with your premise, you missed some key points about this conviction. Most importantly, this was not a case of "freedom of speed". The convicted bared deadly weopons and threatened to kill the victims. The victims of the threats, harrassment, and menacing were mostly innocent children. It is likely that these children will have lasting psychological damage, that gives them nightmares, panic, and anxiety, that could very well last a lifetime.
I will admit that I do take some satisfaction in the sentence, but agree that it is not productive. The trial and any detainment leading up to it were probably punishment enough for the criminals to reflect on their poor judgement and hurtful actions. I suspect they will not repeat this behavior, but then again, their intense racism may be so ingrained that there is no curing it.
If there is one bright spot, it is that this case may bring some of these criminal justice travesties, like excessive sentencing, to the attention of the very people who have cheered for longer sentences in the past. However, I doubt they have the capacity for serious reflection to give it much thought.