Is there any legitimate purpose or utility to the user in that green bubble?
I'm an Android user with no knowledge of iMessage.
If I write a message to someone, them being blue or green means I will either pay for the message (sms) or it's free (iMessage).
And to piggyback off this, to have an iPhone is often a status symbol for teens or some societies. So whether you know anything about iMessage or not, a teen with an Android phone may be heckled when their iPhone recipient sees a green bubble.