What if they don't know they can't fit in the space guaranteed to them? To know this you need to know the pitch used for that particular aircraft, and how that is affected by a seat reclining in front of you. Do you expect people to look this up before buying a flight ticket?
I'm 6'6" and haven't flown in nearly two decades - I have no idea if I'm going to fit into a seat at the moment, and have no idea how to check. I'm facing my first work flight next month and a honeymoon in January, and don't know what to do to make sure I'll have enough room. I know there's seat length available online, but who knows if I'll measure correctly and end up wedged into a seat I can't possibly sit into.
If I were in the top 1% by height (which 6'4" is), then I'd probably have encountered this issue before, and know ahead of time of the risk.
It would be silly for someone in this situation not to look.this up before buying a flight (using seatguru or similar). Just like checking the length of the sleeves is a good idea when buying a shirt online.
While you can check seat pitch with seatguru, a lot of airlines don't allow you to choose seats at the time of booking, and some don't allow booking of extra-legroom economy seats at all. Others charge prohibitively expensive fees for booking the better seats, and sometimes the good seats just happen to be completely taken.
It's also worth noting that there's just not a lot of variety in seat pitch. It's not like I can book a flight with Long and Tall airlines!
Yes, it's difficult, and I wish the airlines provided better information and booking tools.
But my point still stands: if someone _knows_ they are too big for most economy seats, and still buys a ticket which doesn't guarantee a suitable seat (because they don't want to pay the price, or because they don't want to change their schedule to a day when such a seat is available, or because they choose an airline which doesn't let them book seats at all) then it's not fair to put 100% of the blame on the airline.
Don't get me wrong, I'd like there to be a wider variety of seat sizes in economy (maybe smaller folks like me could get a cheaper ticket for a smaller seat!).
