Whoever wrote that doesn't realize that what sie would call a "healthy diet" is EXPEN$IVE. As someone I used to know once said, "When you have $3 to buy groceries for the week, and that will buy 1 pound of meat or 15 pounds of potatoes, you eat a lot of potatoes." Starchy, high-carb diets are associated with both fatness (because they encourage it) and poverty (because those foods are the cheapest).

AKA "correlation is not causation" -- in this case, because there's a third factor operating.