Lots of people have commented on how incredibly stupid these people are. I don't think it's quite that simple.
I think that they're just scared. There's so much fear in our culture, people are scared of health care, scared of a black president, scared of terrorists, scared of oil prices, scared of cell phone companies, scared of pirates (the Somalian kind), scared of pirates (the MPAA kind), scared of the RIAA and MPAA, scared of swine flu, scared of unemployment, scared of having a job that doesn't pay a living wage, scared of peanuts, scared of global warming, scared of pollution, scared of home invasions, scared of floods, earthquakes and fires, scared of nuts with guns, scared of the government taking away everyone's guns.
Fear makes you irrational. It suppresses the "carefully think about the situation" part of your brain, and supercharges the "fight or flight" part. If people stopped to think rationally about it, they would realize it is fiction. But the fear prevents them from thinking rationally.
We live in a constant state of fear, and our culture (or our media, depending on how you look at it) keeps giving us more reasons to be afraid.
What we need is more reason to be hopeful, not fearful. If we remove the irrational fears about health care, presidents, terrorists, MPAA, pirates, global warming, etc., then we would also have fewer irrational fears about the planet Nimbus crashing into Earth on December 21st, 2012.