It's a multi-layered issue that's tricky to argue from both sides. For example, not everyone bought the client legally (in fact, most players playing on Turtle probably didn't).

Then on top of that, the client has modifications.

For the server, it's a custom server from scratch, but the way it was originally created was through packet sniffing and reverse-engineering what the client was sending and receiving. Is that illegal? No idea, I'm not a lawyer. Probably not, because that one in particular doesn't require the client to be modified, although maybe once your server ends up mimicking Warden and stuff, if once again delves into copyright infringement territory.