"Its perfectly ok for the police to batter a man in a wheelchair, or ..."
I think everyone here would agree that the police must not be immune from prosecution when they act in those ways.
"they've managed to pull out a longer sentence for someone who didnt actually kill or maim anybody."
The maximum sentence available for the crime of violent disorder is 5 years according to the BBC, so I can only assume that in the case of those who have ASBOs, either they are juveniles or the maximum sentences are lower for the charges they go into court for.
Again, comparison with other cases on other charges is of limited value - if the sentence seems too short you have an MP and can express your views to them on such errors.
As for vengeful spite - even if he had been given 6 months, he'd have a criminal record and in your words, DeeCeeDee, "fucked for future employment".
Sorry, he is adult enough to go drinking, he would be allowed to drive if he has passed a driving test, he could be married, and a parent. At what age do you think he should be capable of knowing that throwing a fire extinguisher endangers lives ? 25 going on 10, perhaps (or is 10 below the age where a young person is deemed able to know right from wrong in law?)