Just want to point out a few things wrong with your comment. You are using positive and negative reinforcement incorrectly. The terms have nothing to do with good or bad. The terms positive and negative refer to the presentation or removal of a stimulus (reinforcer/aversive).
I know.
You think an overweight kid receiving such a letter instead of candy *won't* feel a negative stimulus from it?
(I see your following post, that apparently you do think such a kid won't feel bad getting such a letter. I can just leave it as - while there may be individuals, I think you're generally wrong about that. Very wrong, even. I expect kids would generally feel shame and anger after getting such a letter in their trick-or-treat bag, which are generally classified in the "negative stimulus" column.)