Where do you get the idea that anyone was "allowed" to attack anyone else?  From what I've read both sides were looking for trouble and I haven't read anything to suggest that the counter demonstrators were any more at fault than those on the other side.  Police didn't take sides, they even declared an unlawful assembly when the counter demonstrators gathered when KKK members were trying to leave and broke up the crowd.  The only person responsible for the death of the young woman was the person driving the car, nobody else.