I'm not sure what you're getting at exactly but it is well known that Blizzard's titles were directly inspired by the Warhammer and Warhammer 40k universes[1].

While the visual similarities haven't causes Games Workshop to complain about infringement from Blizzard (as far as I know) it doesn't mean that Games Workshop isn't complaining about infringement in this or other cases.

[1] http://kotaku.com/5929157/the-making-of-warcraft-part-1

Aside from the(admittedly very close) similarities between the Zerg and Tyranids, there's not that much that Blizzard copied from GW's IP.

However, it's telling that GW's notoriously trigger happy lawyers have left Blizzard alone, so it's pretty much a non-comparison.