Not to say that bad food should be illegal, but you can constrain portion sizes, you can constrain ubiquity (limit the # of fast food places per square mile/per population), you can constrain ingredients, you can constrain their marketing, you can limit how they decorate their buildings, you can do all sorts of things if the public will is there.
Now, whether or not the public will is there is another matter entirely. Policy prescriptions are easy, patient compliance is harder.
Edit: And frankly, I think we're in more agreement than you might realize. I grew up in the Bay Area, and it still drives me crazy how many of my peers would eat Chipotle. McDonalds is one thing, it's bad but at least it's unique, but Chipotle is just bad, expensive Mexican food. What's the point?? But, eventually, you have to accept that people will have bad taste and make bad decisions, and once those bad decisions start affecting society as a whole it's a policy issue, not an individual issue.
You can say it's their fault all you want, but that doesn't actually help you fix it.