I've been overweight in my life and I hate it. You know how I lost weight and kept if off? I ate healthier, I walk a lot (to the grocery store, for example). US grocery stores have all the same healthy foods as every other country. You can eat healthy at restaurants too, if you must.
It's just people don't. There's nothing inehrently bad about food in the US. We at eat too much and don't exericse (walk). Now, if you'll look at all the other countries joining us in this because it isn't a US thing anymore.
You've added all these extra explanations (corn syrup, antibiotics, growth hormones) but it really isn't that complicated. BTW, I'm sure if you ate konbini foods every day in Japan and didn't exercise enough you'd get fat, too. There's nothing different about that food than a lot of junk/fast food in the US, it's just the Japanese seem to walk a lot more and I don't know how many people only eat out of 7/11.