One button.....
Apple believes in the one button mouse from a design point, but there is another reason.
From it's conception in 1984 until today, The Apple Macintosh OS is designed to be the easiests to use OS in the universe, the whole userinterface and interaction is designed arround this principle, the one button mouse fits right into this: All functions within the OS graphical interface are within reach of that mouse, and only one button is needed.
This simplifies the operation of the OS, no confusion on which button to click, REMEMBER this is done for those first time computer users, and is just as valid now as it was in the early eighties. Intuitive control was and still is the buzz word (although it is not "that" intuitive as it once was, due to the enourmous ammount of additions to the core OS today, but most of Apple's software still does a hell of a lot better than most competitors, and is why we all come to love the Mac!)
Those who want to use two buttons because it works faster in some cases, can choose from plenty 3rd party rodents: That said, the only thing Apple could do is perhaps offer one for the Powermac buyers, as an add on or upgrade offer, as these machines will usually find their way to computer pro's and prosumers etc, not your mother who wants to surf the net for recipies etc..
Why not standard, because Apple needs to keep advertising it's ease of use through all their products, and the Powermac being the flagship product, needs to carry the same message to the public....🙂
my 2 cents..