Kevin said (at the time) that normally he buys two seats but this time he had to change his flights for some reason and wasn't able to.
But I personally think he shouldn't have to. Even though I never been big enough to need a seat belt extender and could always get the armrests down even at my heaviest and now I am a normal weight, I still think it's wrong to make someone pay for two seats just because airlines have decided to make more money by making the seats and the spaces between them smaller and smaller over the years.
I agree with the Canadian courts. You aren't buying a "seat" but "passage". Otherwise, why not charge by weight. That way little kids would pay a lot less and tall and/or big people more. Plus, instead of charging so much per "piece" of luggage, they could charge by how much it weighs so those of who fly light can save some money. After all, weight impacts how much fuel is consumed so this way of charging is actually related to costs.
I might feel differently if airlines were told how big to make the seats. But they control the seat size so they are essentially making them as small as they can and then punishing people for not fitting into this arbitrary size.