If you watch the end of the incident you'll see Zidane stumble backwards as Materazzi trips him after the blow.
And no one is commenting on the danger to Zidane in this whole thing. Materazzi hit the Frenchman with all his force directly on his HEAD. In case you are unaware, that's where the brain is positioned. Billions of fragile neurons and tiny arterioles and venules just waiting to start bleeding from such a vicious chest blow. Zidane was lucky not to have had a cerebral hemorrhage. Coup, countercoup and all that. Sure, the Italian put himself at some slight risk of commotio cardis by striking Zidane in the head with his chest, but he counted on the presence of dozens of highly trained German medical professional armed with defibrillators.
The red card, if the incident had actually been seen by a ref real time, should have gone against the Italian.
