Just because this software can be used that way, doesn’t mean you’re required to use it that way.
If I want to start a lemmy server and not let lemmy.world in, there’s nothing wrong with that.
Lemmy.world isn’t owed anything, they’re not owed to view content in my community, they’re not owed that I show their content to my users. And if my users are unhappy with that, that’s fine, it’s their choice to stay in my enclosed community or not.
Just because we’re running the same software and the same communication protocols doesn’t change that.
Maybe the lemmy software doesn’t offer that as a feature right now, but from what I undertstand it’s not an issue on protocol level. So it’s mostly a lack of user friendly configuration options?