Young people about to become smokers are not likely to wake up one morning and say, "hey, I'm going to start smoking today. That display is nice, I'll get that kind." The ghoulish advertisement probably makes them think twice about starting, but that action happens far away from where the advertisement actually is.
I'm convinced it's much more complicated than simple advertising. There's "peer pressure" effects, to be certain. Not the kind of peer pressure that D.A.R.E. warns you about ("hey, smoke this, everybody's doing it. you want to be cool, right?").
If you're a teenager (or college student - I started in college), it's probably more of a network effect. Hang out with people who smoke, and you're more likely to smoke yourself. After a few beers (or other inebriant of their choice), maybe it won't be so bad. Have a cigarette. Huh, nice buzz. Don't worry, I'll only smoke when I'm drinking. Before you know it (I know this isn't everyone - some people can stay social smokers just fine), you're buying a pack in anticipation of a night out. School starts stressing you out, maybe cigarettes will help. And they do, for a short period of time: a few minutes outside clearing your head with a pleasant buzz.
Of course, eventually you don't get that buzz except for the first one of the day. And for heavy smokers, they don't even get that. But it's hell if you try to quit, even with patches, gum, chantix, whatever.
I've only smoked for five years, but I've tried to quit eight times. If people ask me for cigarettes, I don't give them out - I don't know if they're in the starting stages of the addiction or not. And I'm not about to help someone else get addicted to these damn things.