Could they not have made their own? I don't think they'd be required to use open standards for the argument to be made, they just need to release an iMessage app for Android.
There's no valid technical or security reason to do this. It's a tactical decision on Apple's part.
Does it ACTUALLY matter? Maybe not? But people really do complain about a non-iPhone "degrading" a group chat, so it is indicating something.
At the time they made iMessage at first? It was likely a real advance and only because they could control both ends. But now? They may be large enough that it's unfair use of their monopoly in one area to affect another, and get slapped or forced to interoperate.
That might be worth letting the user know about it.