It's more than likely that the previous owner kept the card, probably because they had their home address stored in the Nav. There are ways of reverting that, but I guess they thought it easier just to remove and keep the card.
Having said that, I would have thought that as your dealer was selling the car as a Sport Nav then it would have been their responsibility to make sure that it did have a card in place.
Was this from a Mazda Main dealer or otherwise ?