TEMPE, Ariz. -- Delilah Cassidy's tweet thread was building to a climactic ending. At 1:37 p.m. on May 30, Cassidy, a law student at Arizona State University, began tweeting about her experience at an airport gate en route to Phoenix from a European trip. By now, the story is internet lore. But that afternoon, she was unveiling it to the digital world tweet by tweet. She was about to board an American Airlines flight when the gate agent told her she needed to pay $50 to bring her carry-on with her. If she didn't want to pay, the agent said,...