Well, EH has said "As the stones are in our care, we have to consider whether introducing light onto the monument and into the World Heritage Site will genuinely enhance people’s appreciation and understanding of Stonehenge."
I guess they think neither glitterballs nor Golden Balls will enhance people’s appreciation and understanding of it whereas a massive booze-up inside and on the stones at Midsummer will. It all seems a bit muddled to me. Either it should be treated naffly or it shouldn't, don't you think?