Apparently Blizzard's new engine, called FMOD, by default uses a generic mixer with just a few simple effects. Creative has worked with FMOD to develop a way to run sound through their hardware and enable 3D mixing and other effects (they did this for the PC version of Bioshock, which also uses FMOD), but Blizzard apparently doesn't want any part of it.
So it seems Blizzard treats other hardware vendors the same way they treat their own players-- Creative says they got no advance notice that changes were being made, and apparently requests on why the changes were made have gone unanswered. Of course, Blizzard doesn't have to answer to Creative at all (and maybe they just don't think it's necessary-- I have a Creative Audigy 2 ZS, and my WoW sounds all right, if just a little different). But when we're talking about the biggest online PC game in the world and the biggest sound card manufacturer in the world, it would behoove them both to get along, and make the game sound better for everyone.