Spot on, IMHO. I've never smoked, so there is no personal motive in my opinion on the matter. But to your point, where do we start calling something a sin along the continuum of healthy eating or habits?
This is similar to arguments I've seen where someone takes the position that driving a gas-guzzler is a sin, because of our obligation to be caretakers of God's creation. Some people really took a hard line on that here on CFnet, but it was as illogical as this. Someone who drives a Volkswagen could criticize an SUV driver, but someone who drives a bike could criticize the Volkswagen driver.
Someone who doesn't smoke could call it sin while they're pounding a Big Mac, but a vegetarian could call them out. A vegan could call out the vegetarian.
Sin is ultimately a condition of the heart. It's when we desire to take something that isn't ours or we know we shouldn't. And the sin actually occurs before that act when we have it in our heart to do it. If a smoker is not personally convicted to stop smoking, who are we to call it sin, when we do things we're not convicted to stop?