I don't really get this logic. Why would they want to permanently ban someone paying them money because they also played on TurtleWoW? Isn't the entire argument they're making that TurtleWoW could be hurting their own sales? And given that Blizzard doesn't still distribute their old clients anymore from what I understand, how would you prove that people running the old clients ever even saw the licensing agreement? People can't violate an agreement they never agreed to in the first place. You might be able to make a case for people distributing the client, but in this case that's probably just the same people who ran the server.

There is no customer list. The only thing Blizzard could do is ban anyone using the same IP address as someone on Turtle WoW. However since NAT is widespread in many countries and many people don’t have their own IPv4 address this would result in an extremely high number of false positives. Not to mention that multiple people could be sharing the same internet connection. Besides there’s no reason to do that. Someone who also plays on the official server is paying for a subscription. Banning that person now would just mean less revenue for Blizzard.

You don’t kill a competitor and then ban their customers from your product.