Grenhewe actually hit upon part of it upthread. Positive reinforcement training generally works better than negative reinforcement - you get better results if you reward desired behavior instead of punishing undesired behavior. Specifically, when you use negative reinforcement, you have little control on what the subject associates with the negative stimulus. When you give a kid that letter, they'll feel bad. So, what's more likely - they'll figure that it is the treats that make them feel bad? Unlikely. It is themselves that makes them feel bad? Possibly - so now they are feeling shame and lack of self worth, which is not actually a good way to get people to be healthy. Or, maybe it'll be the nasty, wart-ridden hag who gave them the letter? Also possibly - and thus the TP and flaming bags of nasty substances in her future.
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). So for example, if you were using a positive reinforcement procedure, you would be presenting some stimulus (reinforcer) contingent on the occurrence of a target behavior. Reinforcement results in an increase probability of the behavior occurring, or an increase in some dimension (rate, intensity, magnitude, etc.) of that behavior.
With a negative reinforcement procedure, you remove a stimulus (aversive), contingent on the occurrence of a target behavior. Because it is a reinforcement procedure, it still results in an increase in that target behavior.
Now, while you may think that you are using a reinforcement procedure, and that some stimulus you are presenting or removing is a reinforcer, it may not be the case. This terms (positive/negative reinforcement) are defined functionally. Meaning that the presentation/removal of a stimulus (aversive/reinforcer) results in an increase in the target behavior.
If you are attempting to decrease a behavior, you would use a punishment procedure. Punishment refers to a decrease in behavior. You can use positive and negative reinforcement to reduce behavior.
So, assuming that the letter actually results in the kids decreasing their consumption of candy and other fattening foods, this would be an example of a positive punishment procedure (trick or treat -> receive letter ->decrease consumption of fattening foods).
Finally, you can have very good control of the association in a negative reinforcement procedure.