Obesity is, in fact, a disability if it interferes with a person's ability to interact with the world in a reasonable manner in the way other able-bodied people are able to. If an establishment offers a discount for disabilities, it should not be able to pick and choose which ones it considers worthy of a discount. That's discrimination.
Obesity does limit ones ability to function as a typical person. However, Obesity is more often than not an elective issue. This means that the situation could have been prevented. Very few people who are obese are that way because of a mental condition such as hyper-thyroidism. Even that has been proven to be an irreversible side effect of having poor health habits in the first place. Too many twinkies does not equate to cheaper tickets at the ball game.
Obese fans should not get discounted World Cup tickets for being disabled. In fact, they should pay more because they probably are hindering the physical enjoyment of other guests experiences by having their arms and legs against other people. Being obese isn't a disability, it is just a sign of needing to exercise and eat right.
I do not believe obese fans should get a discount for World Cup tickets for being "disabled." Personally, I do not believe obese people are disabled and I certainly do not think they should receive discounts based on their weight. I think it is far better to discourage this problem, rather than reward it.
No, just because you are obese does not mean that you are disabled in any way. I have seen some very obese people that can get around and work hard. The only way they should be disabled is if they have major health problems from being obese, not just because there fat.