I think it's because iTunes is a windows app as well, and they likely have the same graphic resources hiding in iTunes somewhere. Sucks, I know, but I can't think of another reason.
This is the problem with developing iTunes on two platforms. The Mac version of iTunes on Leopard looks like an old Mac app that was ported to Windows and then ported back again. I just hope they've gotten the clue, and truely unify the UI of ALL of their apps. Everything on Leopard looks so great, but then you bring up iTunes, and it's just sad. And Apple - pick a scrollbar style. I don't care which one you use, but just use one.
One other thing - they have a Windows version of Safari now, but the current Mac version is still a MAC app - why can't the iTunes dev team manage this?