You might want to check ... what was the name of that site with the RO mods? Where Usako lurks... anyway, they certainly must have tons of threads on this kind of subject.
Anyway, yes, it is unethical to simply take someone else's work without asking for permission first. And for the most part, it is something that cannot be stopped (unless you encrypt the grf and distribute a modified hexed that can decrypt it, and pray that you don't become popular enough that someone will get the key from your client and unlock the grf for all to take).
It is the "same" with private servers in relation to RO in general, even though for people it seems easier to justify these kind of things when it is "taking from the big evil money-grubbing companies." Though it might not be so totally the same, as nobody who runs a private server claims to "own" the client sprites and call them their own. But many of the servers who take each other's grf contents do often taut them as their own creation.
