This world is built for the average sized person, and when you are not average sized in any way it's a pain- and yes, sometimes more expensive. I am 4'10", I have to make lots of adjustments in my daily life, from buying step stools for my house and classroom, to having extenders put on my car pedals so I can sit far enough away from the airbag. If I'm in a store, I either have to find someone to help me or climb the shelves to reach stuff. I've learned to adapt and do not expect the world to adapt for me. My boyfriend has the opposite problem. He is 6'4". Like me he has learned to adapt to a world that is not built for him. One example is that both of us cannot just buy any car off the market. When I was car shopping, I eliminated several because I simply could not see. He has done the same thing because it could not accommodate his height.
The same is true of people who are overweight, obese, fat, whatever you want to call it. Sometimes it is by "choice" sometimes it is not. That is really no the crux of this thread. The fact is, the world is built for the average person. If you are not average in size, ability, whatever, you must learn to adapt to the world, not expect the world to adapt to you. In this case, it costs the airline more to fly a passenger who is 400 lbs. They will need two seats which is one less seat for the airline to sell, and it will also cost more in fuel, even if they do only sit in one seat. Either the airline eats that cost initially and passes it on to the customer through more expensive tickets or other fees, or they charge that person more. I vote for the latter.
As for the theme park thing: Aaron and I both work attractions in theme parks. Believe me, we really do hate when we have to ask a person to get off because the restraint system cannot contain them. On the coasters I have worked, we did have seats that were a little bigger and had a modified restraint, but even then some people were still too big. But you can't just replace an entire train with one with larger seats. Not only is that very expensive, but all rides have maximum weight requirements. You would have to go and reinforce the tracks so they are able to handle the weight. Not everybody can ride every ride. Some people can't ride because of disability, motion sickness, etc. Just like somebody who is too short can't ride because it is unsafe, somebody who is too large to be properly restrained cannot ride. In this case, the ride system literally will not allow us to dispatch the train as the restraint is not properly fastened. You still pay the same price as everybody else, including those who cannot ride the rides due to disability. So that is not really a valid comparison.