There are stores here that hand out coin shaped pieces of plastic that unlock the cart that they give out for free. But if i'm not losing my whole 50 euro cent

There are stores here that hand out coin shaped pieces of plastic that unlock the cart that they give out for free. But if i'm not losing my whole 50 euro cent coin that removes the incentive for returning the cart because i've got a unlimited supply of plastic coins so why are we bothering with a lock?