I see this sometimes in Ireland too (on the rare occasion I actually meet people when out and about). Portal tombs generally fall victim to this as they are easy to climb on. Browne's Hill dolmen in Carlow always has tourists climbing all over it and generally about 3 coke cans and few crisp packets in the chamber. Worth than that is when people "worship" at the sites and leave candle wax and things all over the stones...however, that's generally rare in Ireland I find.
Ironically, although I urge people to never climb or interfere with the stones I found a picture of a teenage me stood on the skeletal remains of a passage tomb...I hang my head in shame haha...
I think some us have climbed or sat on long barrows and tombs without actually thinking we were doing any harm. I have a photo of me sitting by the entrance of Stony Littleton which was taken quite recently. More disconcerting perhaps was when I was with some people who were visiting the Avebury area and two of them climbed on top of the Devil's Den dolmen (albeit a reconstruction) - I freely admit I didn't say anything to them but was taken aback when they did it. For me it is an issue of respect as its unlikely that standing on a sarsen stone would damage it.
It certainly is about respect.
There are mixed messages about reacting to monuments. A protester is jailed for painting a green mohican on a statue of Churchill, a famous guitarist's son is jailed after swinging from the Cenotaph, yet BBC Countryfile pull a stunt like putting a big hardboard poppy on the Fovant badges for a day's filming!
Our time is beset with metal theft from memorials and public art, yet is the background to this not just theft ridden hard times but such as TV and charity stunts, protests, vandalism, and for that matter Disneyfying in relation to monuments?
The same goes for clambering over stone arrangements (even those 'restored' in concrete in the 20th century like the Devil's Den) and climbing Silbury in the knowledge that it encourages others to do so and that footfall is damaging the archaeology. We don't know what is acceptable and what is not so we just do as we like.
I don't like praising you VBB as you start punning but that 's a good summary in a lot of ways. We indeed don't know what's acceptable, hence the problem. Even we here don't agree so Joe Public has no chance.
It always strikes me we have no navel gazing about Keep off the Grass signs, people mostly keep off the grass not because the grass will/won't suffer but out of yes, RESPECT for other people or the institution that made the request.
So perhaps we should keep off the bleedin' stones, not because they'll be damaged, not because archaeologists have always treated sites well, not because you've got a wish to kick against authority, but because some people - most people - don't like it and you ought to respect their wishes. trouble is, standing on stones often goes with exhibitionism so asking people not to may actually increase their wish to do it.