I know that from your point of view, it's someone else's job. But let me ask you this: if you are in a grocery store, and you pick up a gallon of milk, but then when you get to the bread aisle, you remember that your wife picked up a gallon of milk yesterday, so you don't need it. Do you just set the milk down on the bread aisle, since it's not your job to make sure that groceries are stocked in the right place (or don't spoil)? Or do you put it back where it goes, to make life easier on everyone?
It's not that it's someone else's job. That's not it at all.
It's that I take offence that companies started putting these up simply to save labor and insurance costs. That's fine that they do this, but I take it as an insult that I am expected to comply with what THEY want.
Apparently this, over time, has turned into a societal norm of courtesy to others, but as a crusty old dude, I ain't buying it.
Hef, using your milk example, I may or may not bring it all the way back to the milk section, depending on how much time I have, but I will at least drop it into the nearest refrigerated (Produce, Meat, etc..) case. I will not leave it on an unrefrigerated shelf.
It's called evolving Tim. They passed out cigarettes with matches on the streets to 13 year old kids in the 50's. Think that's a smart thing to do today? 
But that's completely different. Pretty sure you can't get cancer from parking or not parking your carriage.