No, it's not. People with disabilities can't be charged more for their inherent inconvenience because they're protected by the government. Whether or not that should be so is a moral discussion for a different topic.
Ethnic backgrounds don't cost airlines more money, the people who have actually caused problems do; your extension of "logic" is ridiculous. The grandparent is absolutely right, you're buying an exertion of energy to move mass (your own) to a desired location and buying filled capacity on the plane. It would make perfect sense to cost more for your weight, they just never do this because it wouldn't fly with the public.