If their instance "at's" something, it's (to my understanding) part of their instances programming to post to a community if it exists.
Since a reddit exodus came to .world years ago, a bunch of empty ghost communities exist, so that's why they always land here.
I think there needs to be a clear divide between the two types, for instances that want to do both, have a toggle for how your post should be sent out to the fediverse.