Think about what the hub has to do. It may have to notify millions of subscribers, deal with any errors, retry, etc. This is a very heavy duty messaging system that most publishers will not want to run themselves. And yet you want the publisher's domain name to be the well known resource that ultimately controls things.

Publishers may be blogs hosted on small websites or even things like cars, phones, laptops or home appliances that are not always online or have to work under tight resource constraints.

Publishers may wish to distribute their content through more than one hub. We don't even have to think of avoiding censorship to see why this increases availability.

I think making it possible to split the roles of publisher and distributor is a very good idea. You can still decide to implement both roles on one server.