Taking grocery clerk jobs away

I know people who won't use the self-check "because I don't work there" and "it takes away jobs from clerks because the store doesn't have to hire as many." OK, fine. But ask these same people if they bring their grocery cart back to the front of the store or a carrel after unloading everything, and their answer will be "of course, only jerks leave their carts in the parking lot."

But aren't these the same thing? The clerks are paid to ring up groceries and to collect carts. I used to work in a grocery store, and one of my jobs was to collect carts from the parking lot. Why is it both uncool to ring up your own groceries, and uncool to leave your cart in the parking lot?