http://www.nytimes.com/2013/02/24/magazine/the-extraordinary...
It doesn't take a conspiracy, or especially evil actors, or even the population acting unnaturally (WTF would that even mean?). All it takes is millions of local optimizations to get us into the state we're in now: optimize a marketing campaign for profitability, optimize a snack for deliciousness, optimize what you stock based on what sells and what takes the longest to spoil, optimize what you eat based on price and healthiness judged on limited and flawed information ("low in fat! just ignore the rest of the ingredients...").
Lots and lots of evidence out there for you on the malleability of the human appetite, from people changing the portions that they eat based on the size of the plate they have in front of them, to the absurd effectiveness of rearranging grocery stores to sell snack food, children's food, etc on top of what people would already buy. We are complicated creatures, and you only need a few percentage points from each of these things to justify doing them.
And again, there is nothing evil in saying "I will make a packaged children's lunch that kids will actually love", and there's nothing wrong with a parent deciding to get that lunchable for their kid. We could try to demonize both parties there (good luck with that), or we could try to change the fitness landscape and figure out ways to at least better stack the odds in favor of healthier eating.
Marketing at kids might not be the best lever, but it is one of the easiest, as it is extremely effective (again, see that article), children are already acknowledged as a protected class when it comes to some types of speech (definitely when regulated by the FCC, and narrowly-defined laws like COPA have been upheld by the supreme court, for better or worse), and there's precedent that the public will accept this (parents are already harried enough by requests for specific brands, and the cigarette ad bans in the 90s have shown that there could easily be support for this sort of thing).