Whatever the legal motive, don't you think that "adherent" is more descriptive than "member"? That is, "adhere" means something that sticks, like tape or some other adhesive.
Isn't the real litmus test for a loyal Witness that he/she sticks to whatever the GB says about anything? Thus I can't help but think that any change to "adherent" from "member" has some element of this thinking behind it, whether such thinking is overtly conscious to the GB themselves or not.