AIUI, iMessage was running on legimitate devices. They were using Mac's in a datacenter as a bridge.
I am for reverse-engineering, but at some point civilized society invented the trademark, and today it seems necessary to create similar bodies of law that protect companies from charlatan Middleware that abuse markings of trust for personal profit.
But sure, let's let corporations super-copyright the colour green.
The bubble color was and is a signifier of transport layer only; I've personally had situations (generally with spotty or very poor coverage) where individual messages go over SMS even for conversations on all Apple hardware where the rest of the conversation is going over iMessage.