This is one of the letters TSR sent to anonymous FTP sites in 1994 to force them to remove AD&D related material. It wasn't only meant to remove copyrighted material. Anything bearing an AD&D term such as "THAC0" or "Greyhawk" was infringing, according to the letter, and was meant to be removed. Some people in the AD&D-L mailing list asked TSR which terms were infringing and the response they got was to hire a lawyer.
"A complex system that works is invariably found to have evolved from a simple system that worked." -- John Gall, _Systemantics_