The same reason banks aren't allowed to lend with minimal deposit to low income earners who would happily accept their offer. It might only effect that 1 person but when it happens on large scale it can bring down the whole economy.
I have no problem with an individual wanting to sit in a 2ft box in cargo hold if it cost them $20, but their decision is having a major negative effect on every other normal size person who wishes to fly in some degree of comfort and not pain. Do people not have a right to be comfortable while travelling without having to pay double the price? There needs to be a solution in the middle - not everything is 1's and 0's.
You might say I am denying those with less money the right to travel - I disagree and think there is a solution that will not cost so much more to make this happen. Less money in Wifi more money in passenger comfort. I would rather not have neck pain than watch the latest hollywood film.