Returning your shopping cart might be one of the simplest tests of whether someone has consideration for other people.
Nobody's going to arrest you if you leave it in the middle of the parking lot. There's no reward for putting it back either.
You either take 30 seconds and do it because it's the decent thing to do, or you make it somebody else's problem.
It really is that simple. If you can't be bothered with a shopping cart, I don't trust you with bigger responsibilities either.