This is exactly the same sort of argument Tarmac are spouting. Even though it is well known that Thornborough is a collection of scheduled AND unscheduled monuments within a widespread ritual landscape Tarmac only recognise that the scheduled areas constitute the henges. Thus even though they are destroying the campsites and burial grounds of people who specifically came to Thornborough - they are not damaging the henges.
If they had their was everything we said would be prefaced with twenty words explaining exactly what we mean - result - nobody would read it and they would be more confused by the clarification.
Sorry to use this as a comparison, I'm in no way trying to imply anything by it.
Think of it like this. In 2,000 years time we find a situation where the ancient site of Glastonbury Festival is being quarried. The stages, being upstanding monuments are scheduled monuments and thus protected but the remaining land is being quarried. Protestors of the time would say "Glastonbury is being destroyed" but the developers would simply say they are not damaging Glastonbury - after all, these are scheduled monuments!
In this case the vandalism is something that offends the public view of the monument, and whilst it will probably do no long term damage in itself there are a great many of us who are concerned that this sort of behaviour will expand onto more damaging types, as has been seen at the Roll Rights and Stonehenge for example.