The researchers theorize that the structure of these shows often involved a child committing an immoral action, the child looking cool and strong, and then at the end of the show the child eventually gets a comeuppance (think Harry Potter, where evil character Draco Malfoy is the coolest and most popular kid in Hogwarts and usually gets away with it, whereas supposedly sympathetic character Ron Weasley is at best a lovable loser who spends most of his time as the butt of Draco's jokes). The theory is that children are just not good enough at the whole feedback of conseqeunces thing to realize that the bully's comeuppance in the end is supposed to be the inevitable result of their evil ways. All they see is someone being a bully and then being treated as obviously popular and high-status.

Alternative explanation: They didn't watch till the end? I find it hard to tell from the paper whether they only included cases where they watched the whole way through.