I agree that there is an obesity problem (although I am not an American), but I am not so sure that demonizing french fries, Chicken McNuggets and hot chocolate from age 4 will have the desired effect.
E, to answer the question below: because my personal experience from being a child and caring for a child is that anything that is demonized by the parents becomes extremely attractive.
In response to your edit, there's a lot of gray space between demonizing and letting kids have at it. I was raised on homemade, simple foods and despite loving junk food, I have no problem eating healthy stuff because that's how I was raised. If anything, certain junk food was encouraged, but because the focus was always on a traditional understanding of healthy foods, it was easy to adapt as my personal understanding of "healthy" changed. So we shouldn't demonize anything, but we should definitely teach kids that there's not really any benefit to junk food; if you really have to eat it, make it a treat once in a while but learn to appreciate healthy options.
Anecdote: My son had a close friend growing up whose parents didn't allow candy in the house. Every time a friend's birthday party came around, he would eat himself sick on sweets.
Eventually he learned some restraint but there was a solid 5-6 years there where my son's friend could not be around sweets without ruining his day. Better education combined with a bit of modeling moderation by his parents, might have saved him a lot of miserable evenings.
My anecdotal experience is the same as well. Stingy parents have led to a lot of spending from kid as they become young adults with moeny for the first time.
E, to answer the question below: because my personal experience from being a child and caring for a child is that anything that is demonized by the parents becomes extremely attractive.