What does it mean to "just subscribe to a pubkey" anyway? It means connecting to the relay or set of relays that pubkey has chosen and downloading events from there.
In this case subscribing to a custom relay is different, but not completely different, in practice -- but I grant you we have no way to do this seamlessly currently in any client -- except perhaps Nostur.
It means to fetch and render events created/signed by a specific key. In this case, all article events. The author has no control over who fetches and syncs their stuff. I sync your articles to our relay for example, then subscribe from there (currently via RSS, because I haven't tried out Narr yet). There are also caching relays, mobile relays, etc.. This is why tying anything to a particular relay seems like the wrong approach to me.
Also, relays are identified via DNS and don't additionally sign content, so without any kind of tag mentioning the publication (in that case the relay), the author's intention for being published there is not clear.
