Mastodon servers are separate entities, too. The fact that they communicate with each other doesn’t change that, and the persistent desire that folks here have to imagine otherwise is a hurdle to adoption.
The mental model is of a central space that instances grant or bar access to, but that’s simply not how the technology actually works. Too much effort has gone into trying to make ActivityPub-enabled websites look like something they’re not (centralized social media), while totally ignoring what they are: small forums and microblogs that have optional access to other forums and microblogs.
Mastodon is web server software. “Mastodon” doesn’t exist. It’s an illusion. And the fact that everyone keeps trying to sell this illusion is exactly why there are all of these broken expectations and hurdles.
It’s not pointing out a courtesy, it’s making a demand on a strangers to behave how you want because you want to believe you’re entitled to a safe space in public