Apple doesn't use AMD processors, but that doesn't mean they can't use AMD graphics cards. They were using ATI long before AMD purchased them, there's no reason to ruin that relationship just because of new management. Also, I've yet to see any benchmarks supporting AMD CPUs giving any performance boost with AMD GPUs over nVidia ones.1) AMD owns ATI yet Apple doesn't use AMD processors (which idealy would be most optimal and yes crossfire would be nice)
SLI is not an Intel technology, it's an nVidia one. Since Apple tends to use purely Intel bridges, this means they'd have to go a little further to add SLI. Since the Mac Pro isn't a gaming machine, it's wasted time and money.2) However since they're purely Intel machines now, why can't they stop using ATI and just simply rely on Nvidia for their graphic card needs (Yes the 8800 GT is the most ideal one both money and performance wise and yes if you can put 4 7300GTs [don't know why] then how's about some SLI performance for APPLE!!!
Apple continues to use both AMD and ATI graphics cards because there is no reason not to. PCI-SIG's Express standard doesn't break between vendors, so they don't need to invest anything making their boards work with either brand card. At that point, since some professionals have preference, or even requirements on what brands they can use, it becomes obvious.