I had no idea that cursing was not automatically filtered/flagged/censored on the site, until I had a user flip out on me for downvoting his answer.
Since the site has a strict no cursing policy, how come one of these clean-up methods is not applied?
What is the real problem here, cursing, or abuse?
Abuse
Abuse is mentioned explicitly in the FAQ:
We actively moderate our community, but we need your help to do so. Anything that is getting consistently flagged by our community members will be investigated and followed up on. And of course you can always email us directly if you feel the matter is urgent.
Most importantly, don't feed the trolls! Replying to abusive, off-topic, or inappropriate content only encourages it – whereas flagging allows removal without providing undue attention.
There is already a mechanism present, and in my experience it is highly effective.
If this isn't enough for you, how would you automatically detect abuse in cases where there was no cursing present? Word filters won't work (I'll get to that point later).
Cursing
I don't see cursing mentioned in the FAQ at all, except implicitly as "anything that is getting consistently flagged by our community".
Edit: Cursing isn't allowed, though that doesn't change my final point.
The words the user used in his comments were clear curse words, and there do not appear to be any other meanings they could carry.
You're conflating abuse with cursing. Cursing really isn't bad in a non-abusive context - it is just another way to express one's self.
If someone is angry, they might not be thinking clearly, thus leading themselves into trouble.
As you've pointed out, words don't matter as much as intentions.
But perfect word filters still cannot gather any information on intent. Intent requires AI, and implementation will not be cheap.
Implementation Cost
As detailed in the article Zypher linked in a comment on the OP, word filters suck. Any work on them would be inaccurate and open-ended, and still wouldn't be robust in any sense.
Any word filter would impact site perf. It doesn't matter if it is server side or client side. The server still has to serve up the script, and the processing still will impact end users.
As others have said, the community can take care of flagging if it is needed.
As you point out, there are already mechanisms in place that cover abuse, and they work quite well. Any work in this area would be duplicated effort.
Return on Investment
Anecdotally, I don't see much abuse in general, and very little cursing. Mostly just flame wars and people feeding trolls rather than flagging - and still quite little of that. Our community rocks :)
I don't think we'd get much of any return on the investment, especially since the flagging mechanism already covers most of the cases and already works fairly well.
The Specific Scenario: Our Community
Similarly to how automated phone systems now attempt to predict the emotional state of the caller, based on the way they respond to the automated system.
It might be different if this community were gmail, and were highly prone to drunk-messaging, or were tech support, and people were paying for products. This community is neither of those, so we require different optimizations.
While this suggestion is good food for thought, I don't think our community matches those scenarios very well, so those solutions wouldn't be as effective here.