I used to think this way as well, but I was putting away my shopping cart once and an employee on a smoke break told me, they only get sent outside to put away carts when the parking lot gets too bad -- the boss doesn't feel it's necessary to do it if people still have room to park. And when they get sent outside to clear the parking lot, they can hang out a bit and smoke some cigarettes and otherwise have a break.
I still occasionally put my cart away in the cart return if it's a busy parking lot, but sometimes I just leave it somewhere out of the way of the parking spots. It depends on how happy the employees inside the supermarket seem to me -- if they seem a little overworked and glum, I'm more likely to leave it out of the cart return.