The sentencing seems quite light given what transpired. I assume that this is because the judge felt that Ken Tsang suffered sufficiently vis a vis the cop beating.
If some random dude deliberately poured water/other substances on me or my family with the intention to piss me off, I honestly think that my patience would be extremely tested and I would want a significantly long sentence - a few months at minimum. A few weeks worth of punishment for something so awful is something that could easily be commercialized in the future.
Who wants to walk around a city where people aren't charged for dumping substances onto them?
edit: just read the article - for other offenders their sentencing would likely be much higher so I'm satisfied with the verdict.