Don't forget 9-2-11...
No teammate of a thrower shall be out of bounds after a designated spot throwin begins.
Where 9-2-3 restricts when the teammates of the thrower may touch the ball, this rule restricts where the teammates may be in the case of a spot throwin.
Perhaps that applies to your boundary situation.
Even if not, the restrictions on the two teams need not be the same for both to have a restriction regarding the plane. In the case of the defense, they simply can't be through the plane until it is released. But for the offense, they can't touch the ball through the plane, even after it is released.