With subscriptions attached to hosts, Katello receives a Candlepin event (entitlement.deleted) which removes it from Katello whenever Candlepin removes or unattaches a sub from a host. But we don't do the same for AKs and we never have. There isn't a corresponding Candlepin event (either that, or we don't handle that event) for a sub being removed from an AK.
 
 
so that's why they weren't automatically removed from Katello. It's not going to hurt anything to keep them there, but just for completeness' sake we could add an upgrade rake task to remove them from Katello during the upgrade.