I've got a friend I discuss this with sometimes and we have become convinced this one act (or lack thereof) reveals a lot about a person's character. Returning a shopping cart to its proper place is something one does simply because it's the decent thing to do. There is no punishment for not doing so, and there is no reward for doing so. It only matters to the individual.
This is a pretty fun YT channel on this topic:
This is a pretty fun YT channel on this topic: