My entirely unscientific hypothesis is that serving sizes in the USA tend to be larger, and non-fat foods allow Americans to maintain these larger serving sizes while keeping the calorie density low enough that they don't become obese.
I don't know if I'm right or wrong, but it would be interesting to see a comparison of both caloric density and volume of average meals divided by culture, region, etc.
Actually in my experience low/nonfat foods have just as much, and usually more calories than their original versions. They need to load in more sugar to get a similar taste as the original. Sugar being worse on your metabolism than fat, you end up fatter as a result. Low/nonfat is purely a marketing strategy to convince people they're eating healthy while stuffing their gullet.
I don't know if I'm right or wrong, but it would be interesting to see a comparison of both caloric density and volume of average meals divided by culture, region, etc.