There is a social cost to abandoning your shopping cart; it’s just not borne by the abandoner. Carts left in the parking lot can block parking spots or damage cars if moved by wind or gravity. Additionally, if no one returns their cart, there will be none available at the storefront for use by the next customer. That’s part of the “test” as I understand it - there’s no one grading you individually on whether you fulfill your communal responsibility to return the cart, but that doesn’t mean there’s no impact from your failure to do so.

Feels like we might be talking past each other or conceptualizing the shopping cart theory differently?