It's basically a shorthand way to see if someone is good. Not returning a cart is no illegal nor a big inconvenience. Returning the cart brings no reward but is the right thing to do. In the absence of reward or societal pressure to return a carr, only the truly good will.
Strictly speaking, not putting it away means you value yourself above others. You're taking your own convenience over other people's inconvenience. It's a very minor act, but could fit the definition still.