The shit posters drive a wedge in a community and the people who don't like the "innocent" jokes leave. This makes fertile ground for ideological take over. Rinse and repeat in Fandoms, subreddits, message boards, etc.
Alternatively, the people who are so brittle that shit-posting causes them to somehow turn on half of the community were always going to volatile puritans.
Except it's not so black and white. Crude jokes can be funny and unfunny. Shit-posting is one thing but offensive jokes just often repeat themselves and I can understand wanting something new every now and then.