And where is that written? Do you have any sources, or is that simply your opinion? Now, it is bad manners to shout someone down, and hecklers are usually ushered out of places by security for doing this. However, assault and battery are crimes. Simply put, the heckler was rude, but his actions certainly did not justify an attack; any more than any other protester is worthy of assualt and battery.
I wonder what would happen if a popular Right wing supporter did exactly the same thing to a heckler at a Bush speech. Firstly, the Righ Wing supporter's actions would be condemned by President Bush. And this would be public, and it would be LOUD. Have you heard anything from the Dean camp about 'their guy' assualting a protester? Secondly, the Right Wing supporter would be sued for millions. So, let's wait and see what happens.