Of COURSE Apple could have done it on their own. They could build an app for Android. But they don't want to. Simple as that. And forcing Apple to have to build and maintain an app for a competitor's platform is far outside the purview of the DOJ, and falls very short of "anti-competitive behavior."
If the US Government, or any government for that matter, want to pass a law to force a standard, they can do that. But there is no such law, and there is no mandated standard messaging protocol.
I think Apple SHOULD release iMessage for Android. But having the government force them to do it is silly. And the DOJ is going to lose this argument about "green bubbles" badly. It won't even be close.