These types of messages definitely appear to fit the CAN-SPAM Act, which covers "any electronic mail message the primary purpose of which is the commercial advertisement or promotion of a commercial product or service".

Airbnb may be spending that $100M sooner than they thought: "Each separate email in violation of the law is subject to penalties of up to $16,000, and more than one person may be held responsible for violations. For example, both the company whose product is promoted in the message and the company that originated the message may be legally responsible."

http://business.ftc.gov/documents/bus61-can-spam-act-complia...