It's a restriction on portion size. You can still buy more than one. The human body wasn't designed to handle that much liquid sugar all at once. Those mega-drinks do make obesity more likely. Consumers do not have perfect knowledge. Human's evolved under feast and famine conditions. We live in a society of plenty and it's difficult to overcome human biology even with perfect knowledge. Our physiology works against us. It's very easy to regain the weight lost. I know. I lost nearly 60 pounds but have gained it back. That's over a period of 10 years. I'm in the process of losing it again, but it's going to take longer because I'm older.
There are several questions that need to be answered: Are we our brother's keeper? Do we need a nanny state? Should we pay for the health costs associated with obesity of our neighbors?
Taking personal responsibility is great and all, but it can only do so much. I think a better solution would be a sweetener tax than a portion limit. Congress uses the tax code for social engineering. A sugar sales tax would also be social engineering. By making one of the ingredients more expensive the cost would be passed on to the consumer. The revenue could be used for education or healthcare costs.