That point about cairns "leading people off course" really hits home. I've seen piles of rocks that just mark someone's lunch spot, not the actual trail. Do you think there's a difference between these random stacks and the official, maintained cairns that park services sometimes build?
Once saw a guy stack coffee cups in the office like a cairn, totally threw off the cleaning crew. They thought it was some new system for empty mugs. Makes you wonder why people feel the need to leave marks everywhere.