It may become more difficult (or just less convenient) for the consumer to select software that falls outside of the "Windows App Store" (whatever it may be termed).
This type of distribution model (Steam being another example) would be counter to Blizzard's apparent philosophy on the matter.
So I don't know that "Windows 8", the operating system, is bad for Blizzard. But maybe "Windows 8", the direction, is.
Again, 100% speculation. No necessary basis in fact, just ideas bounced around.