Tell me why.....

[dasher]

After learning of Buster Posey's probable season ending broken leg on a collision at home plate it got me thinking. I  know it is baseball protocol for the catcher to block home plate--- but why is this legal? You can't block a runner at any other base or it would be called interference.
But why is it not only legal but encouraged at home plate?
Umpire Len, Coach Retro, or anyone-- why?

[Sect122Mike]

After learning of Buster Posey's probable season ending broken leg on a collision at home plate it got me thinking. I  know it is baseball protocol for the catcher to block home plate--- but why is this legal? You can't block a runner at any other base or it would be called interference.
But why is it not only legal but encouraged at home plate?
Umpire Len, Coach Retro, or anyone-- why?

Its only legal for any fielder to block a base they are at when they are either fielding the ball or in possession of the ball.  

Here is the rule:
OBSTRUCTION is the act of a fielder who, while not in possession of the ball and not in the act of fielding the ball, impedes the progress of any runner.

Rule 2.00 (Obstruction) Comment: If a fielder is about to receive a thrown ball and if the ball is in flight directly toward and near enough to the fielder so he must occupy his position to receive the ball he may be considered