Free speech basically. It is true that some use such tactics because they can't formulate a real argument or rebuttal. Or some just act in a childish manner whenever someone disagrees with them.
More important is when someone makes unacceptable allusions to violence or other reprehensible behaviours, but I blame more those who remain silent in the face of such things, then I do the weakling who barks in the internet where they don't have to be accountable to anything.
