A previous case established that accessing a web site in violation of its terms of use is not a offense under the Computer Fraud and Abuse Act. That's just a contractual issue.
Here, though, the site operator (Facebook) explicitly sent a cease and desist letter to the accessing service, telling them to stop doing what they were doing (which apparently involved some kind of message sending on behalf of Facebook users). That put the accessing service on notice that they were accessing the site without authorization. After that, access was considered unauthorized. Seems reasonable enough.