They live on the legal fiction that the Client is configured by the user, and the user has the right to the client, so the fact that the client connects to a non-Blizzard server is immaterial.
In the original ruling for the Rebirth case on WoW Bots in Germany, the judge’s ruling was based on the fact that the bots contained zero code from Blizzard and therefore were not infringing copyright.
The superior court’s ruling was that that ruling was in error because without the Blizzard servers, the bots were rendered effectively useless, and the bots were causing material harm to Blizzard’s business.
Private Servers claim that they have zero Blizzard code and therefore hide under the same umbrella. They claim they are emulating abandoned IP with their own work and the fact that they don’t constrain what client connects to them, means that a user using the Blizzard 1.12 client is not the Private Server’s issue.
Once Classic is up and running, the supposed abandonware claim under the Orphan Works Directive is no longer legitimate, and Blizzard can show material harm to its business because they can claim its detracting from Classic subscriptions.
Orphan Works has never been tested against Abandonware however, so Blizzard needs Classic up and running first, to present the strongest case they can. A rushed judgement with a negative ruling would set a dangerous precedent for the gaming industry, and justify private reuse of pretty much any MMO client prior to the current one. A positive ruling would ease the way to reducing their legal burden by sending a C&D with an attached copy of the ruling to every Private Server and waiting for them to cave.
“Yet”.