Websites are indistinguishable from each other except for their content.
Federated instances can be like web service providers, only limiting malicious content from users. The magazines/communities are like web sites on a particular host, such as WordPress. They control the content within their scope.
What you are suggesting is that instances should be like AOL, curating the experience. Which is fine if some of them want to do that, but it shouldn't be the standard.