Personally I don’t look at obesity as a disease. I see obesity as a symptom of a disturbed weight regulation, which is often due to a hormonal problem. Most commonly having way too much insulin in the blood. Obese people often have 5 – 10 times normal levels of insulin. But there are many other possible problems that can also lead to the symptom of obesity.
Thus obesity is a symptom of a disease. The underlying disease is often metabolic syndrome, resulting in too much fat-storing insulin. The cause of metabolic syndrome is often decades of eating too much sugar and other processed junk carbohydrates.
Update: Here’s another interesting perspective on the AMA decision: Wheat Belly blog: The monetization of obesity