It's just Blizzard's pathetic attempt to hold a monopoly over online gaming services.
BnetD getting popular could potentially mean lots of players not on Battle.net.
Lots of users not on Battle.net might take Blizzard out of the #1 online game service spot.
They lose the #1 spot and their ad rates flop to nothing, and they lose a huge edge when releasing new games.
All in all, Battle.net is a SERVICE. Something Blizzard whines and cries about a lot - they want to make sure everyone understands Battle.net is NOT Blizzard and that it is a free SERVICE to Blizzard customers. I don't understand how it's legal for Blizzard to REQUIRE things that prevent fair and clear competition in a corporate environment. IE: They made the game to ensure consumers had NO CHOICE where to play. And they're going to use the DMCA and bogus copyright claims with regards to cd-key checks to protect those interests.
It's amazing to me. I really don't see how what BnetD did was illegal. I also don't understand how what Blizzard is doing could NOT be ILLegal.