It's not arbitrary though. The point was to move away from the metaphor of moving the window around a document to just manipulating the document directly as you do in iOS. Granted it's not as obviously the right thing when touch is not involved, but it is the right thing.
The last time someone really thought about scroll direction was when they invented the scroll wheel. The implementors associated it with the motion of the position indicator in the scrollbar rather than the motion of the document. The reason this is wrong is because it's an unnecessary indirection from the document itself. At the time computers could not documents quickly enough to have any kind of physicality, so the scrollbar + indicator were critical UI elements simply for performance reasons (ie. the scrollbar is what could be drawn fast enough to enable responsive dragging, and the window wouldn't redraw til you let go). Once people got used to it, there was no reason to question it until the modernization of touch interfaces where suddenly the inconsistency became apparent.
While it may be wrong in theory, changing it meant that all muscle memory developed by users thus far was thrown out of whack.
What was the gain for disorienting the users by introducing a new default? An improvement in acceptance by users who had only used tablets and phones but never PC's? Is that a large set?
As far back as 10.3 on my iBook, I had the scroll direction reversed, with a utility that also enabled two finger scrolling on the trackpad.
The "standard" way of doing it never made sense to me, even with a mouse scroll wheel. I scrolled down and the page would go up--how did that ever makes sense?
