That is terrible!
Link for those on HH: http://www.themodernantiquarian.com/post/32340
When I was up there last I had an argument with a gorilla with a great dane and a pushchair. I had the place to myself and was chilling out, but fuming about the big bonfire scorchmark in the centre of the circle.
In walks apeman, saying what a great place it was. I agreed, but said it'd be nicer without the charcoal. He reckoned it was the perfect place for a bonfire. I responded. He joked about coming back that night to have one himself. Before I left I asked him to make sure he soaked himself in petrol, put a firelighter in each pocket and threw himself on it.
Being so close to 'civilisation' it's an easy target. Signs or no signs people who want to do stupid things will find a place to do it.