The thing is that airlines don't always honour it if a big person buys two seats. They might admit another passenger last-minute, and then you paid for two full-price tickets and still have only one seat. It's not like big people enjoy the experience any more than you do. I'm thin myself, but blaming overweight people for terrible airline policies and other things beyond their control is not cool.

See https://twitter.com/i/moments/887384300081098752 for an account of what it's like.