Disney does not sell access to ABC and does not get subscriber fees for it either. Broadcast stations are split up into local affiliates. These are generally owned by companies like Sinclair. Sinclair might chose to require subscription fees instead of listing as a "must carry", but this is completely up to the affiliate and not the broadcast network like ABC.
A better explanation would be subscribing to ESPN without being required to buy ESPN2, ESPNU, ESPN News, ESPN Classic, etc.