OT, Because it's considered bad form to kill your own troops...with your own artillery?
As said, back in 2nd edition you could, there were no rules against it, it was how ever fround apon and any comander willing to bombard his own troops is some one you really wouldn't like on your side in a dubbles match.
Then again if its a foot guard army commander then,
"..I see bravo platoon is being over run down there...Send to artilery command...You heard right, fire on bravos location...Send to Hotel platoon, they are to move up after the shelling has stopped and clear out any thing still breathing and that is not human..."