Personal Choice
America is a country of personal freedoms. Within the confines of the law. And there used to be a time when it was understood that laws which did nothing more than limit the harmless freedoms of people are unexceptable because they encroach on each persons right to choose how to live their lives without harming others. These are the protections of civil rights and civil liberties. A law which limits how much soda a person can drink encroaches on the choose to drink a lot of soda.
This is an example of one person/group not being able to get another person/group to do things their way, so they have the law force them to do it. I'd go so far as to group this kind of law in with laws like "ban homosexual marriage" or the classic eighth amendment alcohol ban where personal preferences are being rolled over by the opinions of a loud, overly-nosey group.
If you want to cut down soda consumption, don't pass laws. Increase public awareness. Create ads and programs to teach the dangers of soda. Don't just tell everyone, "We don't like the choices you make, so we are going to make them for you."
