I've never understood why there's no viable penalty for fans throwing objects on the field. Something other than a school fine - the students/fans could care less whether the school pays a fine or not. But if it were something that would actually penalize their team while in play, they might think differently about their behavior (bottles hitting players, coaches, cheerleaders, camera personnel, etc.). Maybe a 15 yard penalty for interference? If a coach enters the field of play to question or protest a call, they're potentially subject to an immediate 15-yard penalty for unsportsmanlike conduct, right?
Why not the same for fans?