There are two groups of people who it is not just OK to discriminate against them, it's encouraged: smokers and fat people. Why? Well, because it's all
about the choices they make, of course! Smokers can and should quit, no matter what petty silly excuses they make about "addiction". Fatsos should just diet and exercise because we all know they are just lazy and eat too many french fries.
OK, that being said, I can understand where they are coming from. Frankly, I do think it has a lot more to do with cold hard cash than with your comfort. They are coming up with more and more ways to nickel and dime (or by $25-$700) us to death. The cost of oil and gas went back down, did any of the airlines stop charging for the second and sometimes first checked bag? Not a chance! They found a way to milk us for more money.
As long as there are realistic and measurable rules that apply to this, and not some size 0 twig looking at me and saying I'm too fat even though I do still fit in the seat and I can use the seatbelt without one extender (let alone 2), then it's fair. And as long as it is done discretely and not publicly to embarrass someone. I do understand that it is unfair for someone to take up a good amount of a seat that someone else paid for.
I still think it's about the money, though.