Because they punch down while those they police aren’t allowed to punch up. The people being told to ask nicely or argue sweetly are themselves subject to a barrage of hostile garbage. Read Amanda Marcotte’s trenchant analysis: “Tone policing only goes one way.”
Nine times out of 10, if someone is saying something horribly offensive, and someone else calls them out for it, everyone will turn on the person calling them out for it. You see this every time people get more up in arms because someone used the term “racist” or “sexist”, but not so much over the racist or sexist shit that caused the word to be articulated.