There's usually a procedure for reporting lost cards which results in immediate blocking, if you really want to secure those numbers. In Lloyds it's actually pretty strict - I found some wallet one day on a street but without any contact information - called up the bank responsible for the card so that they can contact the owner with my phone number, but they wouldn't proceed before cancelling that person's card. On on hand side I can understand that action, on the other I feel bad for causing that person to request a new card when I was already standing on the street he lives on.
You could have copied the card and months later charged something on it. Someone else than the owner was in possession of that card, it was the right thing to cancel it.
Like I said - I understand why it's done and it seems to be a method of forcing some cards to be blocked ("I found cards with those numbers..."). Unfortunately it causes some issues if you actually intend to return the wallet/card to the owner.
