Let's be fair here. These are chat bubbles. They're not buttons or text boxes. There's no inconsistency in behavior between different versions in different UIs. They're just a pretty way of displaying a chat conversation. It's generally pretty easy to recognize when you're in a chat, and bubble shapes are distinctive. You could even use the tried-and-true text chat approach. This is reasonable as a way to avoid phishing-type scams where you fake IM or SMS to steal information.
The problem is that Apple has decided that people might get confused that they are sending SMS', when in fact they are not. And they are right, some people will get confused, it doesn't take much for people to get confused. And people generealy use visual clues, so if it looks like a duck, smells like a duck, and accepts input like a duck, it is a duck.
