Ok, so it looks like there really are many questions being asked.
1) Is it illegal to host / own / operate a private server for games that require a server of such? And why?
2) Is it illegal to host / own / operate a private server for WOW? And why?
3) Is it illegal to join a private server for games that require a server? and why?
4) Is it illegal to join a private server for WOW?
1 & 2) This depends on the game that is being hosted. There are many games that require servers in which this is not only legal, but encouraged by the company. MineCraft is one that comes to mind. You can host MineCraft servers, and even modify the server code. You can even charge for this service. However,for games like WOW, In short, yes if the server is profiting or If the server is running stolen or leaked software or If the server is distributing client files.
If the server was recreated from the developers own mind, and was only compatible with the WOW server, then it's a bit more gray and depends on the Judge's level of understanding of technology.
3 & 4) This comes down to the EULA. If the EULA is like MineCraft, then you are good to go. If the EULA is like WOW, which forbids both modification of the client and participation on emulated servers then it is a violation of EULA. If WOW did not have the second part of "participation on emulated servers", then a user could modify on the router level to point to a private server.