No, you have it right, but you're missing human nature. If there are a lot of people who want to propagate a rare plant... especially easily accessible ones in

No, you have it right, but you're missing human nature. If there are a lot of people who want to propagate a rare plant... especially easily accessible ones in open-to-the-public gardens... not all people will wait for a leaf to fall. Also, if you're in a private place (such as a plant nursery or a supermarket), you don't have a right to a dropped piece - everything of the shop belongs to the shop, even if you think they'll treat it as rubbish. If a bolt falls out of the display racks, you can't just say "ooh, free bolt on the floor, I'm keeping it", you know?