Man: "I am sorry doctor but obesity runs in the family" gesturing to his obese family.
Doctor: "No, you have it all wrong. The family doesn't run."
Obesity, while fast food is definitely a contributing factor, is caused by the people who do not take care of themselves(excluding special cases).
We all make our own choices in life. If you don't understand that the food you eat can effect you either positively or negatively then you should educate yourself. For those that do understand you have the choice to not eat bad food. No one is forcing you to eat unhealthy.
Although fast food is not healthy for you, there is no way in which you can excuse the obese people from the fact that they made the bad decision to eat it. The fast food restaurants can not be blamed. They do their job properly by making the food consumers want. It is the fault of the consumer if they have too much, and it is not the job or responsibility of the fast food restaurants to tell you, "Sorry, but you're not eating healthy."
It's not fast food alone that contributes to obesity. Though the obvious culprit is the fact that people have become very sedentary, there are also issues with the way our food is being produced. Take the meat industry for example. Studies show that the use of antibiotics in animal feed (in order to fatten up livestock, poultry faster) contributes to obesity.
It is completely your choice in what you decide to eat, if you are obese and do eat at those restaurants, it is not the restaurants fault it is you abusing the food in that restaurant. You really only have yourself to blame as you always have the choice to eat healthy but instead you are choosing not to