Yeah, I just tried your suggestion, and it works like you said. Strange.
After seeing that, I have a similar guess: I'm thinking that during one of the iTunes updates, for whatever reason the mouse-clicking event "forgot" to first check if you're clicking on the active position-indicator bar in the scrollbar or the inactive section. So it assumes that when you click on the scrollbar, you're clicking on the active bar itself. That would explain why absolutely nothing happens when you click and when you click-and-drag, it immediately snaps to the mouse's location on the page and acts like you're dragging the bar.
If that's what it is (or similar to it), I guess we should be grateful that it doesn't get thrown into an infinate loop because of the flawed logic. Based on this, I'm leaning towards thinking that this was a mistake and not just an intentional change/shift in the behavior initiated by one of the programmers.