Apple and Nvidia have done this quiet dance a few times now.
Basically the drivers in Apple's 10.6.8 didn't support Fermi cards but Nvidia released a version that did. Even though the only official shipping Nvidia Mac card at the time was the Quadro 4000 (and still...), the driver contained support for most fermi cards in the GF100 and GF104 lines, from GTX450 all the way up to the Tesla C2050 and Quadro 6000.
This Fermi support got built in to 10.7 when it cam out but we soon discovered that the GF110 and GF114 cards (GTX5xx) could also be supported by adding their device ids to the driver kext.
Nvidia released special drivers for 10.7.3 and 10.7.4 which included this GTX5xx support. Even more interestingly, they added "self init" support. Previously running these Fermi cards required either an EFI rom or Netkas' ATY_Init to match the card up with the drivers. So these special drivers allow Mac users anywhere to walk into their local Fry's or other computer shop and buy just about any Nvidia card and use it with those drivers all by itself.
While the boot screens require an EFI rom, the cards work once the machine gets to the desktop. Using an additional low power card like GT120 or 2600XT solves the boot screen problem if you don't wish to pay extra for an EFI card.
As to WHY? this is the case, I would guess that Apple wants drivers far in advance for inclusion with their OS updates, and that maybe they want to control what cards work and when.
Sadly, even if you run these drivers the OpenCl is still turned OFF for these cards by default. It can easily be turned on by changing 2 bytes in the drivers, so it certainly seems that Apple wants to keep their 5870 as the "golden boy" for FCPX. Turning on this OpenCl allows GTX570 to be fastest card out there for most Mac Apps.