I don't remember the beginnings of such stickers, but it had something to do with the litter produced by the unwanted mail. We have to pay for our litter here. And no one wants to pay for others :)

Just like the same reason for you can get fined for throwing away litter. If it's not your litter box.. :)

I wish that the free willy said "I don't want" is binding for all people over the world.