Is addiction to tobacco a disease? If not, it's unclear why obesity should be. In any case, I've heard this discussion before, but how does defining it as a disease or not help solve the problem?
I don't know about the AMA itself, but most people I know who have sought diet pills found that it was difficult to get doctors to prescribe them, probably because of the potential for addiction. Instead, they tend to recommend calorie management and portion control. Doctors do often prescribe statins, but those are for a different problem that isn't necessarily caused by obesity.