Why did United try to ban Comic-Con travelers from checking comic books?

Travelers leaving San Diego Comic-Con on United Airlines got an unwelcome surprise this weekend, when United said that comic books were banned in checked luggage. United claimed this was a TSA-mandated rule for all airlines operating out of San Diego — only to have the TSA publicly refute United’s claim on Twitter.
As The Consumerist reported yesterday, United posted a notice telling “Comic-Con attendees [to] remove all books from checked bags.” When people complained on