Everyone agrees you should put your cart away, but there is no reward for doing so and no punishment for not doing it. Therefore it says a lot about the individual if they will “do the right thing” just because it makes things slightly better for other people, or if they won’t bother doing a simple act in order to avoid inconveniencing someone else if they aren’t forced to do so.

For such a simple act, it shows an extreme amount of selfishness to not do it.