They just need to clarify--when you purchase airline tickets, are you purchasing transport, or are you purchasing the right to occupy a seat on that transport?
If you're paying to be conveyed to a destination, then it's the airline's problem if they have inadequate facilities to meet your needs. You paid, it's their duty to provide your passage. That they lack the proper equipment isn't your fault--they should have planned better by having more spacious planes, or at least finding out the passenger's sizes before they board.
But if you're not paying for transportation to a destination, but are instead renting a seat on a moving vehicle, then the burden is on you, the purchaser, to purchase the quantity and quality necessary to meet your needs.
So, which is it? I'd say that based on all their rules and regulations, and the industry habit of asserting total control over you and your possessions, and their ability to bump you, move you, delay you, and strand you on the damn tarmac for hours at a stretch while keeping you prisoner, the airlines have clearly demonstrated that their passengers are not renting a seat, they are paying for the service of being conveyed to their destination--the airlines claim the justification for their ridiculous level of control is that it's necessary to achieve this complicated feat of getting you from point A to B. Therefore, the burden is on the airlines. Are people too fat? Then make larger seats. The airline thinks something's unsafe? Make it safe. Widen the aisle. Add more emergency exits. Oh no, your profits? Your problem.
