Anorexia is considered a 'disorder' because it falls within a wide spectrum of diseases loosely classified as 'mental health problems'. Moderate over-eating isn't considered to be a mental health problem but Binge Eating Disorder is - so this form of over-eating is indeed classified as a disorder.
It should be, and I am sure soon will be. Compulsive eating IS a disorder- eating when you are not hungry, not being able to stop, eating for emotional reasons. These are all peculiar to humans and are not in line with how the body is supposed to work. It is th same as being bulimic, only you don't throw up. It is still not "normal" and should be recognised as a disorder.
As the others have said, there are various eating disorders that do deal with over eating or compulsive eating. With Binge eating disorder, most of the root problems are exactly the same as with anorexia.
Compulsive Overeating is also referred to as: Binge Eating Disorder. It is also very similiar to Bulimia Nervosa, but does not involve any purging. Compulsive Overeating is diagnostically characterized by an addiction to food, durations of uncontrolled eating or binging. Post-binge often includes durations of guilt, shame, self-doubt, and depression. Often, Compulsive Overeating is simultaneously diagnosed in tandem with Compulsive Exercising Disorder and can lead to even more serious diagnoses of additional eating disorders.