and being a photographer have spent more hours at individual sites than the average visitor

and I do know the general consensus among the landowners I meet is that left-behind tat in any form is unwelcome, it shows disregard for the animals that may be kept there, it encourages even more tat, it encourages people who show disregard for sites by leaving broken glass and fires burning, it colours their view of allowing public access at all to the point where I often have to make clear I'm only going to take photos and have no 'occult intentions'. Some landowners no longer allow access because they are tired of clearing up 'offerings', everyone gets tarred with the same brush as the thoughtless.

Strangers dont tend to express to me their thoughts on leftovers strewn about but the vast, vast majority refrain from adding to it which demonstrates at least that they dont wish to partake.

I have met many pagans/druids/witches and have gone out of my way to find out more about their beliefs after initially observing a ceremony on the Hill of Tara, I respectfully kept my distance from their ceremonies and declined to take part which was fine by them and me and after chatting to them about a shared interest in Ireland's heritage was given special invitation to photograph more of their ceremonies, not because I was two-faced and pretended to share their beliefs but because both they and I understood that whatever your beliefs or lack thereof, leaving no trace of a visit/ceremony was a matter of consideration and conscience and should be the aspiration of anyone with respect for ancient sites.
I don't have contempt for people choosing their own beliefs, just the actions of a minority who feel their beliefs bestows them with special rights to leave tat lying around.