In many places, you can get fined for littering. Even if it might not be very likely, it’s more punishment than you’ll receive for not returning a shopping cart (unless the place uses carts with inbuilt deposits).

But if i keep my empty beer cans in the car, I can get in trouble for an open container! The law of the land requires me to throw my beer cans in a ditch.

well there is a slight gain from it because i don’t like rubbish laying around, so if i put it in the bin, it’s cleaner.