Problem is, the standards of what is healthy and what is not are constantly changing. Trans fat, for example, wasn't a big issue ten years ago, but thanks to recent studies it's now a required part of the nutritional content labels. These standards also vary from country to country; parts of Europe have far stricter guidelines on artifical additives than the US, and so on and so forth.
More importantly, if you ask me, I don't think entrusting the increasingly incompetent US goverment with setting these guidelines is a good idea. Even if they did succeed in banning certain companies from advertising, the regulations would undoubtedly be filled with inconsistencies and exceptions, and by and large would do little to reduce the amount of junk food american children consume. The buck ultimately stops with the parents, and considering that many moms and dads are morbidly overweight, its not suprising why there's an obesity epidemic in their children.