There are two ways to approach this.
The author assumes that Smith wants to change laws to make oppressing overweight people against the law. This may be the case.
However, the opposite may also be true. That many people feel it is acceptable to annoy, disparage or discriminate against other people because they are overweight.
The airline has a right to deny service to someone who can’t fit in their seat, or who will cost them extra money to transport. But it would also be a good idea for an airline to provide extra large seating at a higher price, as well. There are enough overweight people to do so.
However, if Smith walks up to an airline ticket office, that has no official policy about overweight people, and the clerk tells him that he can’t have a ticket because he isn’t attractively slender, that is a different situation entirely.
If an overweight person wants to eat in a restaurant, and is mistreated by their staff because of his weight, he has every reason in the world to be upset about it. It is not their job to tell him how to live his life, or pester him. They can butt out. And this is not a question of taking his business elsewhere.
It comes down to this. Airlines sell a product. Don't like the product, don't buy it. Its a simple as that.