Everyone's focused on the ethics here, but if this is indeed what AirBnB was doing, isn't it a massive (and repeated) violation of Federal laws on unsolicited emails (for one thing, you're required to identify yourself properly) and likely other rules as well? What this article alleges isn't just questionable marketing practices, it's potentially deceptive / illegal marketing.
Do you drive 66 MPH in a 65 MPH zone? This is hardly breaking the law. This would have been perfectly legal in CAN-SPAM if they added three things: an unsubscribe link, a physical mailing address, and mentioned it was an ad. It probably would have been even more effective.
If you want to stop spam, don't rely on bureaucratic processes to resolve it through legislation. Build a spam filter, or do some simple pattern recognition so you know that the same message sent 100 times isn't a legitimate inquiry to a posting.
No, unsolicited email is never legal, CAN-SPAM only allows you to send email to people you have a prior business relationship with e.g. they signed up for your service/mailing list. Someone who posted a property to Craigslist hardly fits the profile.
