I don't think its ok at all for dogs to jump all over strangers and I don't allow it. They don't need to respond to a stern "down" because it never arises and by the way, to my dogs and many others, down means "lie down" so you haven't even got the command word right.
With many dogs who haven't been taught how to behave, sternness of voice will worry them. Jumping up is rooted in puppy appeasement behaviour so if you scare the dog by your "stern tone" you are just as likely to stimulate it to more jumping as you are to make its stop.
I do agree that its totally unacceptable for owners to let their animals of any description annoy others and that includes the owners of free ranging cats and the parents of screaming children.