Because it's civil disobedience. If you fight back, resist arrest, evade the police, etc it is just disobedience. :)
The point behind civil disobedience is to publicly demonstrate the unjustness of the law, not fight it head-on. Remember when those nonviolent University students a little while ago got pepper-sprayed? Much, much better press for them and their cause than "students start brawl with police".
Some people believe civil disobedience doesn't have to be nonviolent, but most people agree on nonviolence and all the famous examples of civil disobedience were nonviolent (Rosa Parks, Thoreau, Gandhi, etc)