How did Coca-Cola (KO), PepsiCo. (PEP) and their smaller rivals become Public Enemy No. 1 in the battle against America's obesity epidemic? The consumption of carbonated beverages has been plummeting for years. It makes no sense.

But the soda industry provides a convenient target for politicians in cash-strapped cities because it is large and makes a product that is indefensible from a nutritional standpoint. New York Mayor Mike Bloomberg on Wednesday proposed a ban on the sale of large sugary drinks such as soda, claiming the city wanted to do something to fight obesity.  There are a couple of problems with this idea.

First, I don't think it's legal to put these kinds of restrictions on a lawful product. Bloomberg is essentially picking winners and losers in the market because his proposal won't apply to fruit juices -- many of which are chock full of sugar -- and dairy-based drinks like milkshakes, which are hardly paragons of nutrition. Moreover, I don't see how Bloomberg's idea will affect obesity rates one iota. A consumer can buy two 16-ounce sodas instead of one 32-ounce one. What about people who buy large sodas to share with a friend? I suppose they will need to pass a lie-detector test.

Bloomberg isn't the soda industry's only headache.

The Center for Science and the Public Interest, which has long advocated soda taxes to fight obesity, on Wednesday announced plans to hold what it called the "first-ever National Soda Summit," whose goal is to improve public health by reducing the consumption of soda and other sugary drinks. It praised Bloomberg's plan as the "boldest effort yet to prevent obesity."
 
If critics are looking for a war with Big Soda, they needn't bother declaring one, because they are already winning the fight.

As CSPI itself noted in 2010, per-capita consumption of carbonated soft drinks had declined for 11 straight years, a trend it described as encouraging. Sales of diet soft drinks, which aren't that great for you either, are rising, as are those of sports drinks, many of them high in calories. I find it odd that the food police are willing to give these other beverages essentially a free pass.

Any restrictions on the sales of sugary beverages would be bad news for investors and may force companies to increase spending on developing healthier alternatives or buy smaller rivals that produce them. Such moves could eventually yield huge profits for the beverage industry but do little to shrink America's bulging waistlines.

The problem with drink-size mandates and soda taxes is they reduce the very complicated issue of obesity to a simplistic search for good guys and bad guys. People get fat and stay fat for a host of psychological and economic reasons. Making soda more expensive will only hurt the poor and penalize small businesses for selling a legal product.

Besides, many overweight consumers, including me, prefer zero-calorie sodas. The war on soda wouldn't do us much good.

Jonathan Berr is long Coke. Follow him on Twitter@jdberr.