I suspect the effect happens because the photographer is so focussed on taking the shot, they aren't really paying attention to the subject. So in the same time, the non-shooter is actually staring at the art piece. Instead of thinking "I so gotta show my friends this proof I'm in a museum next to some famous dead dood's art"
My wife is a photography person. We have thousands of pics of flowers and pets. We don't even have wedding photos.