Whatever an arbitrary reader of this comment thinks about copyright law -- If they don't defend their copyright, they can lose it. It's kind of a necessity for them to defend it in this manner.
I'm assuming they probably used movie IP in their signage or something. It's shitty, but companies have to defend their trademarks or risk losing them.
The article suggests that the cease-and-desist was related to copyright. A public library’s education initiative should be covered by fair use. It’s unclear what WB are up to except showing their asses.
In any event, if I were Teton County’s head librarian, it would be a cold day in hell before I’d cooperate with WB’s free publicity events again. I’d likely refuse to buy any book they might have an interest in as well.