Offensive Goaltending: WHY?

[rotationslim]

Watched the NBA finals last night, love those Pistons for playing basketball the way it is supposed to be played. (In the interest of full disclosure, I am from Detroit). Anyway the offensive goaltending got me thinking...

Anyway, is there a good reason for offensive goaltending? If an offensive player can get himself in position to guide a shot into the rim, why not? It doesn't make sense to me to punish them?

Am willing to listen to why the rule exists.