The problem with "locking" miles when you make an award reservation is that not everyone has the miles when they make the reservations. For example, I transfer miles from American Express or Starwood into my FF account for an award. I wouldn't want to transfer the miles without being sure I have confirmed an award (or upgrade) seat, and I know that if I call on Saturday and award seats are open, but the time the miles get transferred on Tuesday those seats will quite possibly be gone.