The old macbooks/pros also have the capability of detecting multitouch, it just doesn't have the software. I will be unhappy the way I was unhappy about the special "hardware" that apple put into 3rd gen ipods so they could have shuffle from the root menu rather than from a variety of other places on the ipod.
Has it ever occured to you that perhaps the resolution of the "multiple finger input detection" isn't sufficient for reliably implementing this feature?
I find it hard to believe that if the hardware supported this capability Apple would simply not implement it... As soon as they did so there would be an Open Source hack released to implement it for everyone else and then Apple would look like real idiots.