Of course not all overweight people are like that. But most of them are. If you don't think personal behavior is (in the vast majority of cases) the primary determinant of one's weight, try spending some time overseas. The rates of obesity are far, far lower in most countries outside the US. Just go somewhere in Europe or Asia and look around. Then go to an American shopping mall and compare. The difference is striking. I live in Tokyo, where I almost never see obese people. The rate of obesity here is something like 3 percent, vs. 30 percent in the US.

How do you explain this difference? Do you think only Americans are genetically predisposed to being overweight?

To be fair, in such cases where an obese person really can't control his/her weight (due to genetic or other factors), perhaps some kind of exception could be made. Maybe a doctor's note or something to show the airline that the person really needs two seats. I don't know... just a thought.