A few days ago Continental Airlines announced it would be shrinking its luggage allowance. While perusing this article, I thought of pricing at movie theaters.
Movie theaters price matinee tickets uniformly - if you go to a movie on a Friday night, you'll shell out $10 whether you're seeing The Dark Knight or XXX2. I'd consider seeing XXX2 if it cost less than The Dark Knight.
For airlines, you pay the same amount no matter your weight. I can think of a few girls that pay the same for a plane ticket as I do, even though they are at least 20 kilograms lighter than I am (with luggage, though, we probably cost the same to fly).
Why do you think movie theaters and airlines have flat pricing? A "pay per kilo" model would probably insult airline's overweight customers, though $15 must make a big difference when we're booking plane fare. I'm waiting for the "pay per kilo" airline. You'd buy a ticket with a weight allowance, and pay extra if you and your luggage (together) are over this weight.