AT fire blocked?

[Bluebell88A]

In the assault hex are an American leader and a Para platoon. The Germans have an HMG, armoured car and motorcycle units.

I can understand the reluctance of the German tanks to fire AT through a hex containing friendly armoured cars in case they hit one, but a similar risk of hitting the armoured cars exists for the Americans firing at the German tanks.

Maybe I'm just having a senior moment, but I can't see anywhere in the rules that specifically forbids a unit from firing AT through a hex containing friendly armoured units, or allows a unit to fire AT through a hex containing enemy armoured units.

Tim