This is very different from Beeper or BlueBubbles.

Tether is "merely" talking to your iPhone over bluetooth and the local network. It does nothing if you don't already have an iPhone. Apple should have no more reason to come after this than it does a Garmin watch that can show your iMessages via bluetooth.

As soon as someone starts using this to spam marketing messages over Blue Bubbles Apple will be all over it like a rash.