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.
Read more: https://www.theverge.com/2017/7/24/16021026/united-airlines-sdcc-comic-con-airport-comic-book-ban-tsa?source=Snapzu