You have to look at a modern car as simply an advanced computer on wheels. Like all computers, the maker hopes to make money from combined sales of hardware and licenses for software, including the sat nav. It’s not a rip off, just a business model.
It’s a bit like buying a home printer. The hardware sells more cheaply, in the hope that you will buy more ink cartridges later. Modern cars, with all their proliferation of tech, and expensive safety items like multiple airbags, are ridiculously cheap, just as printers are ridiculously cheap. Not surprising there’ll be attempts to claw back cash later.
But regardless of whether they charge £10 or £1000 for sat nav cards, imho the cards need to be licenced to the vehicle. Else there will always be a theft incentive. Even if the card is stored elsewhere when the car is parked, or even the user doesn’t care about the sat nav, any attempted theft will result from in expensive damage to the vehicle, and trauma to the owner.