"But its only a small minority who let other dog-lovers down..."

No it bloody isn't. It's most dog-owners, who pick up the shit only when someone has seen them, and a healthy minority of those are the tree-slingers. I mean, who'd want to tote a fresh dog turd around for miles? Indeed, who'd want to feel the lovely, soft, smelly warmth of that turd through a thin plastic bag anyway?