Because a 7\seven-year-old girl started having nightmares after seeing it. As explained by the Arts Beat blog at The New York Times, the girl's mother wrote to Harmon in early 2008 -- she knew him through a friend -- and explained the situation. He wrote back, addressing his thoughtful, kind reply to the girl, which is worth reading in its entirety. The gist of it: 'People I didn't even know changed the story.' The out-of-context headline quote: "[Director] Gil Kenan is a hack and [executive producer] Steven Spielberg is a moron." But the heart of it is this: "It makes me mad that it scared you, because I tried to tell them they were making a bad movie that was going to confuse and frighten smart children, instead of making children more brave, and they acted like I was stupid for being afraid that would happen."
Harmon, who went on to create the TV comedy series Community, later "clarified" his remarks on Spielberg via Twitter ("Spielberg ain't no moron, I was just mad"), but it's more proof that writers seem to say they're sorry more than any other professionals.