Sure, but nobody sees cigarette adds, and actually says "I smoke because it makes me look cool". Nobody sees beer ads and says "I drink because it makes pretty girls hang out with me". Yet many many people say "I buy the diet food because I am watching my weight".

Generally speaking, people are pretty good at rationally analyzing advertisements. The ads may still be effective, but almost nobody believes them at face value. Except for diet/low-fat/natural products. It's like people turn off their bullshit detectors for them. I just don't understand why.