If the question were the broader "Who is to blame for declining health in America?", then I would strongly agree with you.

With respect to obesity, specifically, I think very few obese people can honestly say they've been following a healthy diet consistently. Rather, we're talking about people knowingly over eating garbage food. If that's their preference, fine, it's none of my business. However, that also means the systemic problems you point to aren't the primary culprits.