Although we live in a suburb, we are not immune from urban conduct, i.e. graffiti. Trees are spray painted, rocks, the concrete walls of bridges. Note that the vandalism of cactus comes from the Los Angeles Zoo.
Some anthropologists have likened modern graffiti to the rock art of ancient societies. Since we can't know the politics of those societies, it may be an accurate description. I'm not always happy to see graffiti in places that I deem to be unacceptable and I sure wouldn't want it on my property; but there's the difference between modern and ancient times. Private property. Thanks for the opportunity to think about this issue.