I think it's pretty disgusting. As Oron said, it's just a way to make more money, I think it's discriminative and insensitive.
If this would have been any other group (black? old? women?), then there would be so much objection right now that the airline would go bankrupt. Some how, with the craze for young-beautiful-thin-fit, it's almost OK to discriminate fat people (after all being thin is being healthy, right?!)
I can sort of see the logic behind it (more weight more money) but then if I try hard enough I can also see the logic of making women pay more money (the use the bathroom more...) or tall people or whatever.