I doubt the spoilage argument. Milk is a high turnover item (maybe the highest turnover item?) sold mostly in larger containers, meaning the ratio of surface to volume is lower than cans of cola or frozen food packages. The differences in heat transfer between back of the store and moving milk containers to the front of the store must be very minor, not enough to affect taste. Conversely, these days restocking the sloping milk coolers from behind makes the process the most efficient.
IIRC the traditional argument about milk was only part of a larger argument about the placement of fruits and vegetables.