Like, okay - I don't think anyone's saying returning the cart to the corral automatically makes you a good person, it's just a green flag, right? Are you willing to do something that has no reward for compliance and no punishment for abandonment, simply because it's the thing you're expected to do in order to keep a system functioning?
If someone doesn't return the cart regularly, it seems more likely that they're inconsiderate. If they do, it seems more likely that they're decent. But it's one thing out of many that just happens to be publicly visible, so it becomes sort of a benchmark.