They're doing this because you are not to be trusted. They know that you're incapable of deciding when enough is enough. It's because they care. But don't worry! They're here to help you with your problem.
The New York Times calls them "health advocates and public health officials from major cities" (as if an urban address somehow confers gravitas) urging the FDA to take action. They are the Center for Science in the Public Interest (CSPI) and they want you to stop drinking so much damn soda, claiming that "the levels of added sugars in those products is unsafe" and "things like high fructose corn syrup and sucrose [are] a public health hazard."
Their solution? Have the FDA regulate the amount of sugar and other caloric sweeteners that can be added to beverages. Translation: they want you drinking diet.
Clearly this is a subject about which the CSPI feels passionately. They've been campaigning against "liquid candy" since before 2003, when Reason magazine senior editor Jacob Sullum wrote an in-depth article about the laughably inconsistent and unscientific organization. Highlights include a timely section on the CSPI policy of "perpetual Lent" as well as a humorous bit on the supposed gastrointestinal dangers of Quorn, which the organization once hailed as a "meatless marvel."
Perhaps the CSPI feels that New York City's soon-to-be-in-effect ban on large sodas won't go far enough to punish small, minority-owned businesses and otherwise trample on the freedoms of the American people. After all: it is just one city. Then again, maybe they just know they're not to be trusted—and they assume you're not to be either.