If 20 people were undercovers, leading the charge and planting the seed for weeks, it's called "mob mentality" and "diffusion of responsibility". The young and emotionally charged are particularly susceptible and impressionable under such circumstances.
There is the famous Kitty case (that was her first name I believe) in New York where dozens of people were on their balcony watching a man beat his gf to death and not one of them called the police. Why? They were just part of the crowd, someone else surely made a call, I am not solely responsible.
Good law enforcement agencies know how to profile, they dissect peoples motives, human nature, how people operate, they themselves would know that bad things happen in particular climates.
The question must be asked again "is encouraging violence a crime in America"?
I don't think Americans realize how interested in such cases Europeans are. The stakes are very very high.