ATI makes retail cards as well as OEM cards. ATI also makes the drivers for their cards.
ATI doesn't make 'retail' cards for the Mac Pro. Not yet, anyway. Also, ATI is not fully responsible for their drivers (on the BTO optional cards anyway). Apple does help write drivers for some of their cards to ensure compatibility with many of Apple's 'technologies'.
The only reason there isn't retail cards for the Mac Pro is that it's not beneficial for ATI to release a retail card because there isn't a market for one. Apple is selling the OEM ATI Radeon X1900XT and the ATI Radeon X2900XT isn't out yet for the Mac.
This I'll agree with you on.
However, part of the blame lies with Apple for their lack of BTO video card options. That's all I was saying.