HE dumps salt on almost everything, even saltine crackers. He devours burnt bacon and peanut butter sandwiches. He has a weakness for hot dogs, cheeseburgers, and fried chicken, washing them down with a glass of merlot. And his snack of choice? Cheez-Its. Mayor Michael R. Bloomberg has become New York City’s nutritional nag, banning the use of trans fats, forcing chain restaurants to post calorie counts and exhorting diners to consume less salt. Now he is at it again, directing his wrath at sugary drinks in a new series of arresting advertisements that ask subway riders: “Are you pouring on...