The only reason protesters hide their faces is to prevent them from being identified for the any criminal activity they may take part in. Even so called peaceful protests get infiltrated by people who are simply there to cause mayhem and get into confrontations with police because they just hate police. If you feel you're doing what's right, you'd be proud to display you face and not fear arrest. There have been plenty of cases where police depts have been taken to court and had to pay out huge sums to those who were unjustly dealt with. If a cop wants to do something uncalled for, let him face the consequences just like the mayhem seeking radicals. Having said that, I do disagree with events like the clearing of trespassing protesters years ago at a school in Melbourne where police followed protocal and used reasonable force, but were sued anyway some time later and great sums were paid out to those breaking the law.