Some Christian protestors (including Ruben Israel of officialstreetpreachers.com) at the Dearborn Arab Festival 2012 get pelted with water bottles, eggs and even stones (mostly by young kids), and the police stand by and do nothing. The Christians eventually are threatened by the police with citations and/or arrest and are escorted out of the area.
Whether you believe or not that the Christian's were being provocative simply by standing there with their signs, it was their right to do so; and the vicious overreaction by the Muslim kids is not really very surprising given their penchant for anger.
Towards the end, someone on the video tells the police that by forcing the Christians to leave they are rewarding riotous behaviour, and in doing so they're teaching the crowd that all they have to do is riot and they'll get whatever they want.
And that's exactly what they are doing.
Not a great precedent to set.