Fast food tastes good. That's why it's bad for you. Kids don't care about that, and (as of 2010) 18% of children between six and 19 are obese. It's probably no coincidence that researchers have found that these kids have logos "branded" into their brains, activating pleasure centers if they even see fast food.
Researchers at the University of Missouri-Kansas City and the University of Kansas Medical Center picked out 120 different logos, including both fast food companies (McDonald's and KFC) and non-fast food companies (BMW and FedEx), and showed them to kids inside an MRI scanner. Whenever the kids saw a logo of a fast food company, their brains lit up like the Fourth of July with increased blood flow to centers controlling both pleasure and appetite. In other words, according to the researchers, "the brains of children are 'imprinted' with food logos."
In some ways, this isn't surprising: you see something that reminds you of food that tastes good, and you want to eat it. But the real problem is that kids suck at making decisions, and people who sell stuff know that kids suck at making decisions. So, if you can get into their brains and set up an association between your product or service and pleasure, kids will go nuts about it because they haven't had a chance to develop the reasoning or self-control necessary to put those desires in perspective.
It's easy to blame the fast food companies and their marketing for this, but really, it's an end-user issue. Kids don't have to eat fast food to excess, they just want to, like they want all sorts of things that they don't have to have. The fact that their brains are associating the logos with pleasure is a trained response based on long-term experience, and whoever is shepherding them through life should have the ability to either mitigate the fast food experience in the first place, or keep control of it to prevent it from becoming a problem. Until robots swoop in and start raising all our kids for us, anyway.