The food itself is, but you can't hold the company responsible. They are guided by a profit motive. The responsiblity ultimately lies with the decision maker aka; the consumer. The are responsible for their condition, otherwise it is just "passing the buck!"
It isn't just laziness or bad diet. You can also inherit conditions from your parents that make it easier to gain weight and harder to lose. Thyroid is hereditary, diabetes is believed to be hereditary, and so isn't a simply low metabolism.
NO ... the food is not responsible. Neither are the companies.
The ONLY responsibility rests on the parents. They are the ones who allow their kids to eat unhealthy, or to excess ... and let them lead a sedentary lifestyle, rather than exercising.
It doesn't matter how much McDonalds (or whomever) spends on advertising ... or how much the kid asks for junk food ... it is ultimately the parents' responsibility to ensure their child eats properly. Too many parents resort to junk food because it's quick, easy, and to "shut the kid up".
Fast food has been around for decades without kids becoming obese. No, not at all! (in answer to your question) What IS responsible is children, teens, being far less active in daily lives and lazy parents that no longer take time to cook.
Also a big issue i believe is that more and more parents are on foodstamps, whether by choice or by circumstance, and when you have a limited amount of $ for food you tend to make it stretch by buying the cheapest you can. Unfortunately in America the cheapest foods we have available are often the most unhealthy. A bag of apples is almost twice the price of a bag of potato chips. That's a shame.
It's not the food industry as much as it is a lazy society. The fast food industry provides choices and makes it easy to get .. There are still grocery stores, farmers markets, etc that sell the better quality foods. It is still up the the general population to make a choice of what food to purchase.
Part of the problem but not the complete cause. (Fast food companies work very smartly & very scientifically into seducing us to try their foods. It's their job & they do it very well. The only way to avoid their strategies is to not buy into it.) Sedentary activities & lack of outdoor excercise also contributes. Also, too few families sit down together to enjoy a healthy meal, anymore. Half of America (self included sometimes) parks themselves in front of the TV w/a quick junky snack, instead. And too many parents let their kids throw what they like in the grocery cart, instead of choosing healthy food for their children. Usu. to avoid arguments & public scenes. I say, leave the kids home when you grocery shop, if you can. As long as you're paying for the food, you get to choose what goes in the cart. If your kids want sugar sweetened cereals, tell them OK - but they have to pay for it out their allowance. Often, suddenly, Cheerios are not so bad, lol. (Well, worth a shot, right?)
Lack of education is the biggest problem. Food has changed dramatically over the last 100 years but the general knowledge of how it affects us has not been desiminated. Our species has evolved to break down and extract specific nutrients from our environment. This balance has been altered by the introduction of processed foods. Exactly what changes have been made and how those changes affect our bodies is actually reasonably well understood. But few people have been sufficiently educated on the facts. It benefits the companies if people stay ignorant because that is how they make money. It benefits government because they don't have to (or want to) spend money funding another scholastic topic. So as you can see, ignorance is bliss.