Wasn't there another iMessage clone for Linux/Android a few years back and they got a C&D from Apple? Or maybe Apple just blocked their service?
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.
Of course, every spammer dreams of their spam texts coming through as a blue bubble.
The nice thing about Tether is that it’s read only so won’t be attractive to spammers.