Of course the person betting on the fatties would lose most of the time if this game was played repeatedly. You are just stating the obvious.

However, the whole problem with insurance and risk selection is that- what makes financial sense to the insurance company may be grossly unfair to an individual. Let's say you have a body builder whose muscle mass causes him to fall in the 'obese' side of the BMI. He is penalized with a fattie surcharge, even though he is more fit and healthier than 99.9% of the population. Now this makes perfect financial sense for the insurance company- they deal with 100 of 1000's of people everyday. They just don't have time or money to verify every case for every overweight person. For every Mr. Universe they wrongly surcharged, there are probably 20 couch potatoes that they correctly surcharged. It's simply cheaper for them to err on the side of penalizing all 'overweight' people.

In short- it makes perfect financial sense for the insurance company, but you cannot deny that this is grossly unfair for Mr. Universe here.