Because first, way back in Warcraft 2, they added support (by releasing a file) to their game to connect to Kali, the famous gaming network. Which does basically the same stuff BNETD only differently. You get on, find friends, and play a game. No CDKEY checking.

And now they sue these guys?

And lets not forget other Battle.net server emulators, like broodwar.com's emulator, FSGS, GameI, GamePDS amd GameBugs servers.

As a matter of fact, when you log onto Game-I, the very very first line that will popup on the secreen is:
Game-I Battle.net emulator.

Yes, emulator!
How come Blizzard only comes down on one group of people and not the rest around the world?
I dont know, but IMO, these guys are just coming down on them becase they get incredibly mad due to the fact that everyone got a taste of Warcraft 3 via BNETD. But it's not BNETD's fault, its the beta testers who willingly gave out pirated copies of the game.

So yes, I do believe that Blizzard is really trying to pull of something excessivly lame.