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.
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!
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!).