This might be in the D&D FAQ, but I'm a bit too lazy to check it right now.
It seems to me the player's guide doesn't have a lot of information about how evil Clerics rebuke.
When you rebuke undead, and they are "cowering in awe", do they get to attack? (assuming the cleric who is rebuking them isn't the one attacking them). It simply says in the PHB that the undead cower in awe, and that you get a +2 to hit them or whatnot. It doesn't specifically say that the undead don't defend themselves.