It was a massive misuse of power by an adult over a child because it was done to punish and humiliate, without any authority and with an audience - the child was 10 and was laughed at by onlookers throughout and made to sweep up his hair afterwards.
If your hairdresser shaved your head with clippers set to number 1, to punish you for cutting it yourself, and forced you to sit in front of a laughing audience to have it done and sweep up afterwards you might also feel violated and call the police.
However I totally agree the sentence is not proportional to those received by sex offenders and killers.
A jail term for a misuse of the adult/ child power imbalance to deliberately and publicly humiliate a child in a fairly ritualistic way, especially a child over whom you have no legal authority, doesn't seem intrinsically wrong to me.
However the jail term seems far too long - it's only needed to make a point that there are consequences. In fact a hefty fine would have been much more appropriate probably, but what he did wasn't ok, especially when you read how he did it.
The issue with the sex offences and killings is that they did not receive appropriate sentences, obviously they are infinitely more serious.