I know in the comics it was "Zorro" that inspired Bruce. Was it a "rights" issue that prevented Zorro references in the Bruce Timm cartoon? Or was it simply the needs of the story, someone mimicing the crimes depicted in the old serials that resulted in a slight change in Bruce Wayne's backstory?
I think it was Frank Miller that introduced the Zorro thing in either The Dark Knight Returns
or Batman: Year One
, most likely as an homage to the fact that Bob Kane was influenced by Zorro in creating the Batman character. Earlier comics (and the first screen adaptation of Batman's origin, the Super Powers Team: Galactic Guardians
episode "The Fear" by Alan Burnett) had established that the movie the Waynes were leaving at the time of the murders was Robin Hood
. So it's not like "inspired by Zorro" is an inseparable part of the Batman myth.
Apparently the copyright status of Zorro is disputed
, but it's probably in public domain now. But a judge ruled that the copyright in the original works "lapsed in 1995 or before," so the copyright may still have applied when "Beware the Gray Ghost" was made in 1992.
However, I doubt that was a factor. The episode was written as an homage to Adam West himself. The Simon Trent character was specifically based on West -- a skilled actor who was famous for playing a classic TV hero but whose career largely stalled afterward due to typecasting (though Trent had it much worse than West). And probably they made his character an original creation rather than something like Zorro because that way they could shape the idea to fit the story they wanted to tell.