Extreme content is whatever most people don't want to see. Nazi flags are legal to display in the US, but most people and (more important to Twitter's bottom line) most brands don't want to see them on their phones when they open the app. Charmin doesn't want its toilet paper ads next to posts with Nazis or gore videos or hentai or many other things that are legal but not something users want as part of their user experience. People won't open the app as much or not sign up to begin with, twitter will get fewer ad impressions, and their revenue will plummet.