That's correct, there is no consistent policy.
Cairns certainly have a scenario where they can be helpful, like on trails systems without many signs. SATN is a great example where those visual makers are helpful.
Another good use of Cairns is were the absolute Cairn cities would (or do they still? ) pop up when the BC Greenbelt creek was dry. Dozens of stacks artfully places 6+ feet and taller. And like sandcastles, the water removes them eventually.
But stacking rocks that'll just stay like that for no purpose? It's like, if you're going to graffiti, make something beautiful out of something grotesque (like on many SATN structures and overpasses, instead of just shitty tagging). So I lean to the "leave no trace" side here when it comes to nonsensical reorganization of roks. Not today, hippy!