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?

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[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 “in the act of fielding a ball.” It is entirely up to the judgment of the umpire as to whether a fielder is in the act of fielding a ball.

If you catch the views of plays at the plate from the few above cameras left in mlb, you will see that usually catchers do not block the entire plate until they have the ball.  If they did, obstruction would be called.  Catchers are taught to block the top half of the plate and give the runner the back half of the plate to slide into or run over.   If the runner runs into the catcher when the plate was open as I described, the umpire should call the runner out for interference and probably toss him from the game.  Usually the plays run smoothly.  Either the runner slides and tags, or steps on, the back part of the plate before the catcher has the ball.  Or the catcher gets the ball first and clearly blocks the plate with the ball and braces for the incoming collision. As long as the catcher can set down low, the pads help him take the momentum the runner has.  Those big hits happen when its close.  IMO, many players at all levels think of the play at the plate as an exciting part of the game and one where a big shoulder check should be employed.