One of Apple's core design philosophies is that when something is not in active use it should be hidden from view. Through this perspective it makes sense that they would hide scrollbars when the user isn't doing any scrolling.

That being said, I would rather have it always be visible because I'm not that much of a minimalist and I still think that non active information has value at a glance.

Edit: This doesn't mean Apple tries to hide everything that isn't in active use. It's a design convention, not a law.