It is important with any interface to have feedback for actions performed. In mechanical interfaces, that comes through physical motion and tactile feedback. With electronic devices, almost all interaction takes place through the manipulation of some on-screen rendered image of an interface which represents something that is easy for us to navigate. When we’re using a keyboard and mouse or trackpad to navigate an OS, we don’t get that tactile feedback. Sure, the keyboard and mouse in and of themselves are physical devices, but they click or “press” just the same, regardless of whether the action you performed on-screen was successful or not. Because of this, it is crucial that the interface provide feedback of some sort. This is exactly what Apple does with the elastic stretching when trying to scroll past the end of the page.
My previous laptop, a Samsung 7-series, uses the Elan smartpad. Unfortunately, it doesn’t pick up finger touches as well as other trackpads, and so sometimes I have to repeat my attempt to scroll in order for it to register. Since Windows doesn’t perform any action when trying to scroll past the end of the page, there are certain times when I’ll be looking through a page, reach the end of it, and when it fails to do anything, I think that the touchpad simply failed to register my action. Windows doesn’t provide any feedback when I am unable to scroll anymore, and so I have no distinction between “end of the page” and “didn’t register scrolling attempt”.
When you reach the top or bottom of the page and you can’t scroll any more, OSX does this little elastic-stretchy thing where it’s as though you’re trying to pull the end of the page up or down, but it’s stuck. This elastic stretching provides the user with feedback. It’s a way of saying, “hey, I know what you’re trying to do, but you can’t do it.” Even if my MacBook Pro had this problem with the trackpad not reliable picking up gestures, I would be able to differentiate between a failed scroll attempt and the inability to scroll down any further.