People do seem to have an irrational bias toward backpacks. I was going to a Michael Moore speech a few years ago and I got harrassed by the cops for trying to bring in a backpack. But it was only backpacks that they were interested in. Not any other kinds of bags. They also refused to check it and instead suggested that I hide it in the bushes. They also threatened to arrest me. If I had gotten a big shopping bag and put my backpack in it, I would have had no problem.