The USB2 controller (EHCI?) is pretty much 'independent' of the USB1.x controller (OHCI/UHCI) despite being on the same chip, so it might be sort of reasonable that you don't get "Hi Speed" support until the OS actually supports EHCI. (Since the 1.x controller works, at least things get to work at all, rather than leaving you completely dead in the water as may have happened with last decade's standards.)
It might also be sort of reasonable that Via didn't cut their own driver for the EHCI end of things (or at least, all the code required to create one) at least until MS did their share of the work in making XP handle the 'standard' in some sort of hopefully-consistent way.
That's the entire extent of my understanding, maybe someone like Platon knows more?
Edit: So for curiosity's sake, I hit up Via's site, and find "Due to licensing agreement, VIA USB2 driver package requires Windows XP SP1 (Service Pack 1), Windows 2000 SP4, or system upgrade through Windows Update." Well, that sure is crystal. (Licensing agreement for what?)