It's the same as XMPP/etc vs proprietary chat protocols. "Slack chat is only for Slack customers." "Google chat is only for Google customers." And so on. We

It's the same as XMPP/etc vs proprietary chat protocols. "Slack chat is only for Slack customers." "Google chat is only for Google customers." And so on. Well....if you say it is, then it is. But the supporters of interoperability would like to...interoperate. So non-Apply customers can send messages to Apple customers and vice versa.