Obviously, since the comics are offline, we have no idea what material Blizzard objected to -- you could argue that depictions of certain gear created by Blizzard artists in the game or specific names are under trademark and thus could be protected under copyright law. But even then, Shakes and Fidget is one of many, many fan-made webcomics to obviously depict World of Warcraft and Blizzard-designed items specific to the game, and we haven't yet heard of any other webcomics that have been approached in this way.
Still, as they might say in Germany, wir riechen eine ratte. Without knowing the exact nature of the comics that Blizzard had an issue with, this seems like a complete overreach on their part, especially considering that it's some of their biggest fans who are making these comics, and that many artists have portrayed many videogames in webcomics without anyone confusing the issue of whether they were official or not. We'll keep an eye out for more information (and we've also contacted Blizzard for comment) -- if you know of any other webcomics or fan artists who've been approached by Blizzard in this way, definitely let us know.