Airbourne player charges into backward moving defender
 

Lets say: B1, in a LGP, slowly moves backward in anticipation of A1's move. A1 jumps forward and toward B1. B1 continues to slowly move backward until airborne player A1 charges into B1.

According to the rules, this should be a foul on B1 because B1 obtained his legal position (the position at the time of contact) after A1 has left the floor, 4-23-4.b. A defender who moves backward after the opponent has jumped towards him is obtaining a new position after that opponent has left the floor. Isn't that a violation of rule 4-23-4.b?

No, see what B1 is allowed to do after gaining LGP.

B1 hasn't lost LGP by moving backwards...if a call is to be made, then it would be a PC/charge call.

I understand what you are allowed to do after gaining LGP. Heck, I understand that B1 has done nothing illegal. But it's just how the way the rule is written. "If the opponent with the ball is airborne, the guard must have obtained legal position before the opponent left the floor". To me that sounds like once the opponent has left the floor, you can not move to a new position.

You're reading it wrong. All that means is a player can't legally place himself in the path of an airborne player if he wasn't already there before a player is airborne. Per your OP, the defender already placed himself legally in the path of the airborne player and by virtue of having LGP (per your OP), he's legally able to move backwards to maintain it.

He obtained it before, and maintained it after. It also says the defender may move to absorb imminent contact.

I believe he is hung up on the word position, thinking it means a single position on the floor, as opposed to a position relative to the opponent.

He also might be thinking that the shooter's right to land includes a right to the entire path to that spot, whether or not a defender was in that path first.

If A1 would have jumped completely OVER B1, and then B1 moved to a new spot to cause the contact, you might be right.

In the real world, see the other answers.

David Thompson with the Nuggets???