[crazy voyager]

Correct fiba mechanics for this is that if the shot comes from the leads area is that the lead shows "3-point attempt", and holds that signal until he can confirm the trail has mirrored the signal. He must how ever still cover the rebounding and therefore most of the times when this happen you see the lead holding the signal until the ball has either rebounded or entered the basket (if the ball enters he should look at his partner to confirm 3 points have been awarded.
(From the Fiba 2-referee manual - shooting situations- three point attempts [6.3])
The reason the lead won't score the basket (I think, I have no quotes on this) is becuse the trail is responsible for the flight of the ball and therefore he is the logical choice when it comes to scoring the basket (since he sees when and if it is good).