No helmets on should mean not confrontational in Police Body Language - it both shows the cops aren't armouring up and showing faces makes them more human and less easy to project anger upon by the protesters, meaning less escalating behaviour from the protesters side. The Swedish police recently went through a redesign of their riot gear to make themselves look as unarmoured as possible, based on the theory that the animosity between cops and protesters/rioters causes a circle of escalation of the conflict - protesters are angry, police is scared and becomes angry, protesters gets scared and angrier, and so on until cobble stones and gas grenades start flying and the armed forces starts stomping people. The idea is that because the riot police looks so much like faceless soldiers it's very easy to dehumanise them, and by making themselves look as unprotected as possible (without actually being unprotected), and thus not as threatening, the protesters will be less likely to attack them (in short, it's easier to throw stones at someone in armour and wielding a giant shield than someone without). Psychological warpolicefare ho!
Of course, none of this applies when it's the police who's doing the escalation, as from what I understand has been the case with (most of? All?) the Occupy protests (I don't even know if any of them degenerated into actual riots and clashes with the police as opposed to just being brutalised by them). Just a bit of random stuff I felt the need to share.