I think it's the usual Mac argument for bespoke ROMs and drivers: They have a limited range of hardware with a limited number of combinations of said hardware.
As such, they can test one set of drivers with relative ease, so that they ensure they work 'out of the box' for any user.
If they let you download the vanilla drivers, which are untested on their hardware, then it could cause complications, and they don't want Apple to be seen as substandard because of a third-party.
That's the theory anyway – whether it's true in practice is another question.
You could argue that as you're running Windows instead of OSX it's not their problem, but then they pride themselves on Bootcamp improving the usefulness of their product, so it makes sense to not let you break it even if it isn't their operating system.
tldr; I've got no idea.
