There's nothing stopping nvidia from theoretically offering a gpu that is only rented out rather than sold. It's just not really considered acceptable for hardware (at the moment).
That's only because the hardware is a useless dongle without the software.
Sure in theory you could run an open source driver, and in practice sometimes the river won't crash, but there's no point because you could get an equally good open source driver video card for the same price, since you can't get the fancy card's peak performance from the open source driver
