The problem for the rangers is that they are not able to risk a violent confrontation with these people, and police resources are stretched too far. It might have taken a couple of hours for the police to reach the scene and by then the perpetrators would have fled. Far better to offer the photographic evidence to the National Trust now and suggest that they might have enough evidence to make a prosecution, or at least send a very strongly worded letter informing these people of their disgusting behaviour.
Something is not right. This message is just to keep things from messing up down the road