Over the last couple of years I've actually been guilty of leaving the trolley in the carpark if I'm in a bad mood, which happens a lot mind you. I always make the effort of putting them back but there has been about 3 times I've left them. I feel guilty about it after I leave but it's mainly cause there isn't a trolley station anywhere near by.
I think a few more bays could alleviate issues of people dumping them where they park. That's from my point of view over here. I've gotten into the habit of as we walk past a bay and cause there is only 2 of us, I load up and carry all the bags to the car while the mrs puts the trolley back. Seems to be working well lately. Obviously that wouldn't work with people with large families and more groceries.
I always feel bad seeing the little Indian worker having to scourer the carpark collecting runaway trolleys. He probably doesn't get paid much so if I can make his job easier I'm more than happy to do so. Sometimes as he's collecting I'll take the trolley over to him to save him walking. They always seem very appreciative of this and that's a good thing.
I think putting them back is just more satisfying from a personal point of view. I guess it's a moral thing. I've done it as I said a few times but I don't anymore.