The CFAA says that "having knowingly accessed a computer without authorization or exceeding authorized access [is a federal crime]".
The courts held that 3Taps scraping the Craigslist website was accessing a computer and exceeding authorized access because it should have been obvious to 3taps that craigslist did not authorize them to scrape their website (namely from some IP blocks and a C&D letter), so it stands to reason that talking to the iMessage API from a non-apple device is a federal crime. Apple has only authorized apple-devices to talk to their API, it should be incredibly obvious to all of us that this is not being done with apple's authorization, hence crime.
In case it's not clear, I think the CFAA is a rather poorly thought out law since "authorized access" seems like it could be as vague as a ToS violation, which means it escalates things that seem more like civil matters into federal crimes.
