I think the SMS fallback makes this fair. Apple saw how bad SMS is, they made it so iPhone-iPhone convos use an upgraded mode or fall back to SMS automatically, and I don't see why it's their job to give the upgraded mode to everyone for free. You can still talk to Android users in the same app.

They even put early effort into making Macs send/receive SMS via the iPhone. Google didn't do something similar until recently, or did they, hard to tell with their chat app naming.