Why do people joke about it? Many reasons. Because it's human nature to joke about things. Because there are about a billion people flowing through the system, joking about everything under the sun, and odds are that at least one of them will joke about a bomb, just through sheer numbers. Because a natural reaction to absurdity and security theater is to make fun of it. Because when I tell you "don't think about a white elephant; think about anything but a white elephant", you naturally start thinking about white elephants -- and so if I tell you "don't joke about airport security; joke about anything but airport security", it's easy to inadvertently tell a joke about, yup, airport security.