In situations where mob mentality can take over and chaos can create a "fog of war" this is not true! Even though most of the protesters want to remain peaceful, the amplitude of sowing chaos, and its potential to spark additional chaos within mass rally situations, are providing extremists that "single point of weakness."
Basically, against an organized opponent that wants to create chaos, mass rallies are kind of like packing together certain isotopes of uranium atoms. Even if most of the units are "inert" (in that most of the people want peace,) if enough individuals with activation potential are densely packed enough, you can still get the chain reaction.