> where are people expected to learn these implicit rules? If the store doesn’t care enough to communicate these expectations (assuming they even have them, and that they don’t only exist in the minds of the self-appointed “cart police”), why should customers follow them?
The rules are not implicit; there are typically giant signs saying "RETURN CART HERE" over a metal cart corral that often contains other carts.
People are expected to learn this during their first or second trip to a grocery store that offers carts.
Similarly, at a full-service restaurant, you will be able to notice busboys picking up used tableware, and you will notice a scarcity of customer-accessible garbage bins (as compared to, say, a self-service fast-food restaurant).
If you are ever unsure of the protocol, you are always welcome to ask an employee. Employees at these businesses are typically distinguished by wearing a uniform.
Hope these tips help you on your future trips to Kroger/McDonald's/Olive Garden.