The fediverse is not private. It is so non-private, that I’d say it’s hovering somewhere on the opposite spectrum. That’s ok, because it was not built for privacy. It was built to democratise online spaces, and it does that very well.
Self regulation is possible on the fediverse because a semi central authority (instance admins) can choose to defederate from other semi central authorities (instance admins); this still doesn’t really silence anyone since they can form their own instance and do whatever they want in there.
In person to person chats, self regulation does exist - if you don’t like what someone says or does you just stop communicating or associating with them. If what they’re doing is immoral and illegal, you can report them yourself. What private communication is about is that someone disconnected to the conversation can’t come around snooping without the consent of anybody actually in the conversation.