Apple seems to avoid interoperability wherever they think they can get away with it.
Even in really weird ways, like: to install any software on your Apple phone you had to use Apple's music player. (Is that right? I never owned one, and it sounds daft, but plausible for Apple.)
Text and video messaging was low-hanging fruit, thanks to XMPP etc not taking off. With that network effect in place, it's a short leap towards a subtly-incompatible text format, so buying Apple's stuff becomes the path of slightly less resistance.
I don't know whether this is an aggressive strategy; or whether they care only about their customers' satisfaction and not at all about interoperability.
