I don't think these protocols are quite comparable to a Twitter-like "town square" model, so they don't have the same problems. But where they overlap, they do some "censorship": SMTP will delete spam, XMPP servers will kick out spammers and harassers.

The problem with Twitter is that unlike RSS, it shows content from people you did not subscribe to (e.g. you get replies, you see strangers' replies to your friend's posts). The content is public, so unlike e-mail, receiving abuse is not just your private problem, but a problem for the network's image and reputation.

But in either case, when normal users (not righteous keyboard warriors) find some content to be horrible, they don't want to see it. If they keep being bombarded with unwanted content, they leave. For Mastodon/Twitter/Gab that may be "political" content, but the problem in general is similar to spam.