No matter how you slice it, and despite the fact that something like 1/3 of Americans are obese, there is a hidden view of overweight people that they are somehow morally weak, and that their overweight state is an outward reflection of that moral weakness. Hence, they "deserve" to be "punished" by having to pay extra for an airline seat.

From a purely economic perspective, it seems fair but let's not delude ourselves that there isn't a moral judgement component involved as well.

No, there really isn't. It is simply a question of fairness and civility. The simple fact is that if you can't put down the armrest when you sit next to me on a plane, you are intruding on the space I purchased and I am going to be physically uncomfortable for the entire flight. Fortunately this doesn't happen all that often, because even most "obese" people actually fit in the seats, but when it does occur it is truly a miserable experience.

Agree with Texvol. Any judgement of obese people is completely irrelevant here. It doesn't matter is they're weak morally or physically. The point is that they are infringing upon the comfort of others - others who have paid for for a seat and are expected to fit in it.