Jack Meserve identifies it:
[T]hink of a few of the currently illegal vices: recreational drug use, gambling, prostitution. With some exceptions, the left has been in favor of legalization or decriminalization of these activities. Now think of legal vices: gluttony, cigarette smoking, alcohol use. On these habits, we’ve supported bans, onerous restrictions on place and time of consumption, and increasingly aggressive fines and taxes. There seems very little consistency between these positions, and few have even attempted justifying the differences. Progressives have been guilty of letting our temperament rather than our reason guide the policies; bans on activities like drug use are seen as naive or old-fashioned, but legal vices like cigarette smoking are public-health or collective-action problems to be solved through brute government action.
He thinks a form of "cultural elitism" is to blame:
Someone who buys a 20-ounce, 330-calorie Starbucks cinnamon dolce latte is viewed differently than someone buying a 20-ounce, 290-calorie Mountain Dew from McDonald’s. The latte would be allowed under Bloomberg’s ban, the Mountain Dew not. Similarly, marijuana smoking has a cultural cachet that cigarettes have lost. In fact teenagers now smoke pot more than they smoke cigarettes.