The relevant element to me is less the rational construct behind my actions but what the social contract and environment expects of me. I can violate those rules but then have to live with the consequences. In cases such as this I prefer peace over conflict. It’s a small price to pay for me to return the cart and what I gain is not to cause unhappiness in those around me and thus in myself. This is reason enough for me to do it. In other cases, I might want to demonstrate that I do not agree, but I try to be conscious in what fights I want to invest my energy in.