What is "the community's right to shut somebody up"? If a country elects a dictator, it is fine if that dictator then shuts people up? Mastodon (afaik) already

What is "the community's right to shut somebody up"? If a country elects a dictator, it is fine if that dictator then shuts people up? Mastodon (afaik) already has the feature that communities can decide what outside content can enter their servers. What is the point on making it impossible for others to use the software at all? It doesn't server the purpose of protecting that community from nasty speech, because that is already accomplished by not communicating with the evil servers.