It’s just bad policy, and counter-productive. You could test this, I suppose. Here’s an article that says that no extrinsic motivator (neither reward nor punishment) will help people in the long run. They can have short term effects, but as soon as the extrinsic motivator is removed, they go back to old habits.
The article suggests that only programs that train people and support them to engage in healthy behaviors have any effectiveness (the example is about smoking, but it’s not a big stretch to believe it applies to weight loss as well):
By contrast, three better-designed experiments – in which various kinds of training and support were provided for quitting smoking – discovered that the effectiveness of these interventions was reduced if a reward was offered for kicking the habit. In some cases, people promised money actually fared worse than those who weren’t in a program at all!
There seem to be articles all over the place about weight loss motivation, some suggesting that both positive and negative reinforcement work. However, almost all do not seem to be backed by research, or the research is suspect. Many protocols do not follow people over the long term.
This article also says that negative reinforcement doesn’t work. It suggests ways to develop intrinsic motivation to lose weight.
There are many other articles out there that suggest that extrinsic motivators have only a short term effect, and that the best way to motivate a person to lose weight is to help them develop intrinsic motivation. There are many programs out there that claim to do this.
Anyway, it seems to me that taxing fat won’t work. I guess paying for weight loss won’t work either. This leaves us with at least one policy alternative—teaching folks how to maintain a desire to lose weight; how not to beat themselves up about it, and what techniques they can employ to help them.