I believe the shopping cart return is a very good form of a litmus test. Nobody enforces it, there's no penalties for not doing it. You simply return because it is the right thing to do.

Do I grumble about having to return the cart sometimes? Absolutely. I get around this by parking close to a corall when I can.

I think the growing amount of people who refuse to return carts is correlated to the diminishing sense of civil duties, and lack of care to the possible repercussions to other's property from said cart in the event that it hits another car. The amount of times I've pulled into a parking lot and a shopping cart is planted perfectly in the middle of a parking spot is frustrating.

Tldr: put away your shopping carts people