@croydon - I don't know if weight-loss is the best example though. 50 years ago there wasn't such a large obesity epidemic going on and that really is something to be afraid of if you are overweight.
I know this is about using the power of persuasion to get people to buy, rather than being done in their best interests, but I don't actually see that there is a moral issue. If they have no reason to be afraid, then the advertisement isn't going to work. If they do, then maybe it will help.