The scene was violent and chaotic: protesters being forcibly removed from a rally for Republican presidential candidate Donald Trump. The image reinforced a common complaint that police can be callous toward the free-speech rights of demonstrators when seeking to preserve order. […] … A large gathering mixed with the natural political friction of an election season, or any event that draws crowds in the thousands, can be a volatile brew. Any such event, from a political rally to an NFL game, needs police present, said Jon Shane, an assistant professor at the John Jay College of Criminal Justice in New...