Is obesiety genetic, or is that an excuse?

I understand that some genetic traits can cause a person to be obese, like thyroid problems, but is a large part of it diet? I can't imagine that somebody that eats low simple carbs, lots of fruits and veggies, and meats that are low in saturated fat could be obese. Then shouldn't people not have the right to blame their weight issues on genetics? Think about it, when was the last time you saw somebody in a 3rd world country that was obese? and in america, if your obese then, it might be your genetics?, so by coincidence the country with the worst eating habits in the world also has the highest rate of genetic obesity. That is a suspicious statistic.