As opposed to leaving the cart anywhere in the lot for other people to traverse and maneuver round. It's not about morality testing, saving money, or stopping theft. It's about safety.
Yes, there is a moral component. Leaving your chart anywhere is like dropping your trash where you're standing. It shows you disrespect others and their property.
Social mores maintain social order.
I'll grant that the solution is better than the alternative. But I resist the notion that courtesy and morality are synonymous.