It is not a tradition dating to Walt, more dating to Eisner. Watch the Walt Disney Anthology Series and you don't see Imagineers wearing name tags. The term Cast Members did not start right away and was more a public term. Even into the 1970s there are Walt Disney World documents referencing "employees." It also makes no sense to call everyone a Cast Member. The term is supposed to be like in theater and film, but you don't see the accountants, writers, director, lighting crew, etc. listed under "Cast" on the credits.