Nah, I think it's that they don't want people to know that Shiek is unlockable yet. If there's a new move there, then people would guess at it, but if nothing is there, people will just assume that Zelda can transform into Shiek. There's no real reason to hide Zelda's down-B attack otherwise, since she's most likely in the starting roster, by virtue, if Shiek were the down-B attack, Shiek would be in the starting roster as well, and as far as we can tell, we likely know everyone in that starting roster now. The reason the move is hidden is to both prevent people from thinking Shiek is out of the game and to prevent others from figuring out Shiek is a hidden character, I bet. There's just no reason to keep the two together, all it really does is give them one less move than everyone else, since there's no balancing act similar to the PT's stamina. Even now, Zelda's appearance is from a game where Shiek doesn't exist. As it stands, it just limits the characters to one less attack apiece.