Man oh man, there are a lot of (very bad) armchair lawyers on HN.
You might consider AirBnB's actions unethical, but they certainly aren't illegal - which would be obvious to anyone who bothered reading the CAN-SPAM wikipedia page. It's not illegal to violate Craigslist's TOS, and AFAICT the "spam" messages were sent from a legitimate Gmail account.
For those of you claiming "I'd rather be poor than violate craigslist's terms of service": Bullshit.