Blizzard can look at the code and claim theft, which they haven't. And source code is hard to get your hands on since that requires someone to hack into Blizzards servers, which I haven't heard anything of such theft.
Or people making strawman to defend their arguments.
Storing Art would be a horrible inefficient use of bandwidth for Blizzard. All servers do is tell the client where everything exists in the world via X/Y coordinates with probably a Z thrown in as well. Add a chat system with SQL server to store players info and you have a WoW server. No art assets were taken here.
Something for people to think about is that piracy is usually a symptom of a bigger issue, and not a core problem. I always like to bring up the MP3 and Divx codecs as examples of legal tools that were always associated with piracy, and is responsible for the creation of Netflix/Itunes and similar services. The technology has improved to the point where people can do something that needed a whole company to do. Companies don't like to adapt to the new demand, and end up like BlockBuster.
In between that mess, a lot of companies tried to make MP3/Divx illegal, by making ripping illegal. It was challenged many times in court, but in the end ripping music and moves from Discs is not illegal. Nobody has challenged if a WoW server is illegal in court, and yes this is important cause if it is breaking any laws then Blizzard has the right to shutdown private servers. Which would definitely make people rethink of playing on these servers if they can be shut down at a moments notice.
This is also relevant to other games today, like Battlefield 1. Don't know if anyone's heard but players can't make their own servers. Instead EA wants you to rent a server,
which isn't sitting well with the community. Don't be surprised if someone makes software to run private BF1 servers, given there's enough people interested.