"If a child is obese, then the obesity is a problem. Even if they got that way from eating healthy foods, and even if they do hours of exercise a day. confused"
Not necessarily. Visceral fat, nutrition and fitness levels are the issue.
Visceral fat is the fat found around your organs. Generally it's what gives people a chubby belly, although often it doesn't show at all. Fatter arms, legs, bums are Subcutaneous fat that is stored directly below your skin and is the type of fat where you can “pinch an inch” from your belly, arms, thighs or just about anywhere else on your body. A lot of where and how easily you gain subcutaneous fat is down to genetics.
Visceral fat is dangerous. It's linked to Type II diabetes, cancer, high cholesterol and blood pressure, Alzheimers, etc etc etc. All those life style diseases related to obesity are actually all related to high amounts of visceral fat.
Generally, if a person has a lot of visceral fat then they have a belly, but this is not always the case. There are lots of "skinny fat" people have a lot of fat around their organs, but are a healthy weight. Those people are at just as much risk for lifestyle diseases as someone who is obese.
Also, someone who gets fat eating lots of chicken and spinach is going to be healthier than someone who gets fat eating Big Macs. Because there is so much more sugar, salt and bad fats in Big Macs than there is in chicken and spinach.
Exercise has been shown to be incredibly beneficial in so many ways, that I can't begin to list them here.
However, its a hell of a lot easier to say that fat=bad, rather than an unhealthy lifestyle + genetics+ fat in certain areas =bad. Especially since for much of the population, if you are obese, then chances are you are living an unhealthy lifestyle.