I always "share" my spot (especially on the squares but not always in the outside shopping malls) however my "cart" has regular doors and my "courtesy" often leaves me climbing in on the passenger side crawling over the protruding parking brake lever between the seats which can be quite painful if you sit down too soon
due to the fact the person "sharing" with me always pulls up (or back) too far and blocks the very obvious doors. I try to park on the back of the spot but they still always seem to block my door.
That being said there are no published rules requiring space sharing or even published courtesy rules requiring space sharing. If the "selfish person" who wouldn't get back in their cart and move was still in the cart I could understand the OP's frustration but since they already left their cart I wouldn't be so upset and need to make a public issue over it but that seems to be the way it goes here.
Sometimes you just have to suck it up and move on with your life and not let all the little stuff bother you so much. There is way too much BIG stuff we should be concerned with that goes on.