What about trolls?
Won't someone think of the...No, wait.
Given the number of vicious trolls I've been encountering lately, (just mention any hot button topic), one would assume the trolls are going to figure out how to overload this with false positives pretty quickly.
So even if you think they're trying to do a good thing, it will fail.
I recall a time when "do good" was more important than "don't get caught" but it was a long time ago.