It is partially to blame. Simply by existing and selling tasty junk food they make they are part of the problem. Much of the time it is (but isn't necessarily, there will always be self-defeating fat people) a mixture of clever marketing and weak-willed "victims". Stressed out parents go there to buy a quick meal for their kids with the marketed "healthy options" or other such excuses at the back of their mind to appease their conscience. Then they buy a meal with a diet coke and think "oh what the hell, I may as well have a sundae, I deserve it".
It's not just fast food. If someone regularly eats regular McDonalds meals and lets themselves get fat, the odds are that they buy shitty supermarket food too. You know, stocking their trolley with fat free noodles, sugary tomato sauce, coke, hormone-fed cage chickens, "lite" ice-cream, falling for every health-related marketing gimmick even when they half know it's probably BS, and maybe throwing in a bag of frozen vegetables to make themselves feel better.
There are fast food restaurants here that actually sell vegetables, but you actually have to go look for them because they're not shoved in front of your face via corporate marketing campaigns, and perhaps *gasp* even step out of your car to buy it and maybe even persuade your children to eat that unfamiliar food that doesn't even have a toy or a colorful box or a sugary drink with it. I didn't even like McDonalds at first when I was a kid. I ate the fries, kept the toy, had like two bites of the burger, and coke tasted gross to me.
