Similar to a Carpet Bagger, a milk bagger is a person who is hated by the general population, and does change the milk. Said "Milk Bagger" is someone who will drink the rest of the bag of milk, and they will put it back in the fridge. This person must be dealt with by being given the asshat treatment.
Joe just chugged the rest of the milk and shoved it to the back of the fridge. What a dirty milk bagger.