In or Out of Bounds??

[easygoer]

This has come up several times. On a play under the basket near a sideline whatever. "A" player who is in the game steps out of bounds with both feet, "B" player (of the opposite team) trying to save the ball throws it off of "A" player who is standing out of bounds. Whose ball is it? The argument is that since "A" player who has both feet out is out of the play, it can't be out on him. If this were the case what would prevent the defense from stepping out of bounds on every saved ball attempt and touching the ball while being out of bounds??

Thanks