Perhaps, but I think the main reason horse collar tackles were singled out a few years ago is because of the injuries that accompanied them. Typically, these occurred when a player was sprinting down the field and got caught from behind with a horse collar, and it was that type of situation that caused the injuries. It's probably less of a concern when a QB is within the 15-yard "pocket", since he's not likely running (at least, not full speed).
All that said, it does seem strange that they wouldn't have just outlawed all horse collars, and it seems like they should just do it.