It sounds really smart and self-righteous to say 'it is the parents' responsibility.'
Clearly none of you work directly with this problem of obesity.
After years of seeing obesity rates, diabetes rates going up, if you are trying to deal with the consequences of it every single day, you sort of get beyond the 'it's the parent's fault.' Of course it is the parents' fault. But saying that does not help to solve the problem. You guys must all live in well-educated little bubbles. The reality is that the parents of many of these young kids are not well educated, are obese themselves, whatever. It really doesn't matter if I yell at them and tell them it is their fault, though.
As long as the companies like McD's keep on marketing like this, people are going to keep on going. It is obviously an effective marketing strategy, since their stock price keeps going up and our society's health is going down the drain.
Someone is going to have to deal with it, sooner or later. Believe me, if you are a working taxpayer, you are going to be paying for this problem. You can either try and deal with it by regulating things like advertising, or you can smugly sit on your hands and be self-righteous and grin and say it is the parent's fault.
I can give you a few other arguments and you can see where you want to draw the line. How about a law saying that if you live within city limits, you have to put a fence around your swimming pool. Well, let's say I'm Mr. Libertarian and I don't like fences, so I protest and don't put up the fence. I also like toys, so I put lots of them around my pool. The neighbor's toddler sees them, comes over, falls in and dies. Sure, it is the kid's parent's fault, but the kid is still dead. Is this really all that different from luring kids repeatedly into your burger joint with toys to make a buck?