It's a mix of a few factors... product placement payments and availability. When you see PCs, it's likely product placement. When you see iMacs and iBooks, it's either product placement or availability. When you see PowerBooks and PowerMac towers, it's availability because they usually have so many of these on set (editing suites and whatnot) that they just grab what's available and put it there. The PowerPC series of Macs are quite popular in the editing industry.