I don't know, you'd have to read the study. Point was they found a strong correlation between ads and quitting. Obviously the ads were communicating health information, so it's weird to say that the ads were the reason since it was likely the health information in the ads. It's also quite possible your friends feel more comfortable citing the cost over the health impact, since they would have known about the health impact when they started. "It got too expensive" is an easier out than "I used to not care about my health, but I do now"