I often wondered why eating fast food and being overweight isn't a disfellowshipping offense? The logic behind the smoking prohibition is that smoking is harmful to your body and your body is supposed to be a temple to Jehovah. Therefore, putting high levels of sugar, sodium, and saturated fat would also be the same as inhaling tobacco smoke.
I remember as a kid my friend's dad, who was not a born-in, getting disfellowshipped for smoking. I also remember going out in field service on Saturdays taking our morning break. Chances are it was a McDonald's, a donut shop, or some other place that the members of our car group would intake huge amounts of sugar and sodium.