The shift to the sinful stuff has created a butterfat shortage, driving up the cost of the ingredient, and therefore the cost of ice cream itself. The drop in low-fat demand has caught companies off guard. Meanwhile, Dreyer's will be raising it's prices 10 cents to 30 cents a half gallon in some markets because butterfat prices are at record highs.
Higher demand is only half the equation.
Supply also is down. The quality of feed also was down, and cows produce less fat in their milk when they don't eat heartily.