I disagree with returning the shopping cart being an act of free will. There is a lot of societal pressure to do it for some people, or to not do it for some other people. And there is always the risk that someone who you know will walk see you not returning, and tell all your friends about it. Or want if your boss happens to see you? What would happen then?

So yeah, better quickly return it. Better than having to deal with all these unknowns.