First, because the use of the word can be in a positive or negative sentence, and thus can either agree or disagree with your "censorship rule". You need a deeper NLP analysis to disambiguate.
Second, because - if online fora with this kind of ban has taught us anything - it's pretty easy to work around it by using punctuation, replacement characters, or other style variations to write the word in a way that it won't be caught by the regex.
Third, this method ends up banning legit words. There was a forum where I couldn't write "booby trap". I don't need to explain.
WRT reputation, networks that allow "unlimited registrations" are easy targets for https://en.wikipedia.org/wiki/Sybil_attack - One can use account age to mitigate of course, but given the amount of fake accounts that exist on the market (look at fiverr marketing offers) with long life and activity, I wouldn't expect age to be that big of a helper.