Well there are many competing princilpes, and the way we prioritize them will affect the kind of society we end up with. I believe in this instance, the benefi

Well there are many competing princilpes, and the way we prioritize them will affect the kind of society we end up with. I believe in this instance, the benefits of equality outweight the costs. If we believe that a "free" market would distribute prices differently so that lighter people paid less and you believe the free market price is fair. Then some small ecnomic harm is occuring, to those who pay more. The social cost of rectifying this small harm however is, i think, considerable. It makes transportation harder for poorer people and perhaps those that need transport the most. And so on. In this case, i think the $500 extra /flight can be distributed across the couple hundred passengers without much to-do.