The problem is, if Blizzard knows of an infringement and fails to attempt enforcement, anyone can come along and legally claim "Hey, they didn't hassle this (small time) guy, so they have no right to hassle us". So the well-meaning hacker guy with a cool prototype opens the door for Zynga to come in and make StarCraftVille, which I'm fairly sure would be not so well received.
Blizzard has a legal right to be dicks about their IP, but that doesn't make it morally correct. We've refused to let IP holders set the narrative with regard to DRM; let's not do it here.