I completely agree about not leaving items, such an unthoughtful thing to do.
I disagree with you on concerts and lights, and think its good in this case. CADW were involved and I strongly suspect they wouldn't leave anything behind or damage the monument. There is a debate to be had about modern usage of monuments and I wonder if we ('modern' people) actually now have a right to reuse them? Nobody owns these sites anymore (except us through CADW), and historically they would've been reused by communities who had in some cases no continuity with each other. I'm thinking of my local site - neolithic and then reused by a Bronze Age community a thousand years later! So perhaps artistic/cultural reuse (even for a concert) can be justified.
But I get where you're coming from. We must protect all of these sites as they are still enjoyed and appreciated.
Something is not right. This message is just to keep things from messing up down the road