It the pressure in the sealed container that keeps it from freezing. Even non-carbonated beverages like Gatorade and Hawaiian Punch have pressure inside the bottle. The bottling lines for non-carb plastic packages have a liquid nitrogen system that puts a drop in each bottle right before the capper. This turns to gas and stiffens the closed container so they can stack the cases on the pallets for warehousing and shipping without the bottom cases crushing bottles under the weight of the load.