1) always return the shopping cart when it's free (it almost never is)
2) rarely return the shopping cart when it's paid - sorry but I value my time more than €1 it cost to rent the cart, and, well, clearly there's no "social contract" - there's an "explicit contract", which says "you rent the cart for €1 and we refund you if you return it" so clearly not returning it is fine (also, someone could earn €1!)