I don't mind if they just leave it in the parking lot, there are employees who will come around and gather them up.

The problem is when I find a Walmart card in the middle of nowhere

Here in Philippines its expected for you to leave it be at the parking spot. Someone from the Mall/ Supermarket management will return it. There is someone doing a dedicated job for it. Not sure if it's a bad courtesy in here. But you would be probably robbing someone's job for it. The same is also with going up and down with elevators. Though not common in office buildings. I guess this probably started in USA as a cost saving idea for the companies, similar to how they convince us that jaywalking is bad and not tipping is bad.

I return my cart if they have returns conveniently placed near where I am parked, but otherwise my feelings are well summed up by Zorg.

Hypothetical: do you leave your triplet infants in the unsupervised car to return the cart a minute away as to not be deemed an animal by people that didnt see your kids, or do you return the cart then try to carry three infats back and put them in the car? Not everything is black and white.