It seems like all of the films set in modern day cities have an error, the people never know what zombies are even though zombies have been in popular culture for some time. Even parodies like "Shaun of the Dead" never touch on this phenomenon. Why wouldn't they know and if they did wouldn't they have to toil with myth and fact I think. It would make the stories that much more intriguing.
It's genre blindness (Warning! TVTropes link!). It's not just zombies, that happens in many genres. For example, when there's something frightening out in the woods, why do young teens split up in small groups, have sex, then wander about all alone, with the women wearing almost nothing? You'd think they'd know better because it's always when you're alone and often after you've had sex that you get killed in some gruesome way.
It also works for long running TV shows, like in Star Trek IV, when they arrive in San Francisco in the 1980s and Dr. Gillian Taylor hears Kirk's communicator beep and she doesn't make any Star Trek jokes, or for people in London who see a blue police call box and don't start making jokes about Doctor Who.
Essentially, in most cases, people have a blindness to the genre the film or TV show is part of. But there is also the exact opposite of this, genre savvy (Warning! Another TVTropes link!). This is when the characters show an awareness of what happens in movies or TV shows that are similar to their current situation. And they often point out all the connections to the tropes that show up in their situation or say, "I told you so," when someone comes up with a genre blind plan that fails.