"Encouraging shittiness" because they're pointing out a lack of feature parity with a color seems like a stretch. Google has done the same exact thing by pushing a number of chat apps in their ecosystem, the only difference is that Apple has succeeded.

It's not a stretch at all. They are well aware of the weird culty green bubble clique attitudes. And there are plenty of things they could do to allow imessage on Android. And even if they still didn't want to do that, they didn't need to choose an entirely different color for non imessage messages, they could be more subtle in alerting iphone users the other person in the convo has less features available to them. Much of the time it isn't even relevant.