Via Reason, Russ Smith opines on how some products get stigmatized with "sin tax" status while others don't:
Why stop at soda? How about a tax on every calorie-laden coffee drink served at Starbucks and its competitors? After all, a vanilla bean frappuccino with whipped cream is more than 500 calories, a beverage that health researcher Mike Adams calls "dessert in a cup." Throw in a scone or brownie with one of those Starbucks "desserts" and a consumer is approaching, at mid-morning, the daily recommended calorie intake . . . [New sin taxes are always] aimed at the déclassé products, such as soda and fast-food burgers . . . If it's true . . . that "we" would be thinner and richer by laying off sugary drinks, wouldn't the same apply to the more upscale foodstuffs consumed every day? After all, obesity knows no economic boundaries.
Why is government in the business of taxing whole product groups arbitrarily? Incentive-wise, a sin tax only focused on soda does one of two things: (1) makes people most likely to purchase those drinks spend more money (possibly making it a highly regressive tax) or (2) enhances the profits of corporations providing substitution goods which deliver similar benefits. You're essentially picking a winner in an industry. If you really want to tax obesity-creating products, why not tax by calorie?