A state law cannot pre-empt a federal law; when a federal law and a state law are in conflict, the federal law takes precedence over the state law. (U.S. Constitution, Article VI, Clause 2.) Notably, the CAN-SPAM Act expressly pre-empts any state law that "expressly regulates the use of electronic mail to send commercial messages, except to the extent that any such statute, regulation, or rule prohibits falsity or deception in any portion of a commercial electronic mail message or information attached thereto" (15 USC 7707). (So, a CA law against fraudulent email would survive pre-emption by CAN-SPAM, but a CA law against simple bulk spamming would be pre-empted by the federal law.)

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.