I am just pointing out the obvious here, but doesn't the recent study that showed people who regularly each chocolate have lower BMIs run counter to the food reward theory? If there was ever a food high in food reward, I think chocolate would be it. It is both high-fat/high-sugar, and is used in cakes, cookies, candy, etc.
I noticed at the grocery store the other day that one-half of an entire aisle was devoted to chocolate products. That is a ton of shelf-space, and therefore that means there is high demand for the product.
My take on this would be that chocolate itself is not the problem, it's the bingeing/overeating of chocolate that is the issue. If chocolate can be eaten when hungry and consumed to satiety, then it may not be a promoter of weight gain. In this study, perhaps the people who consumed chocolate frequently learned to regulate their consumption. Others who like chocolate but occassionally binge on chocolate products may instead gain weight.