Good question. The answer is simple, they don't have to. Copyright comes under the banner of IPO intellectual property ownership.
What is copyright? One has to ASSUME that there is an owner of an image, and it is YOUR DUTY to find the owner and ask their permission. EVERY SINGLE IMAGE in the world is owned by someone. The owner might well have granted unlimited use of their images, but it is YOUR duty to find out who owns the image before use.
I feel for you I really do, but believing you have the right to use someone else's property simply because you do not know who the owner is, or that the owner has not written " this property belongs to" on it is naive in the extreme.
I have suggested MANY times that Getty could overcome this by watermarking ALL their owned images. Hell they do it on istockphoto, so why not elsewhere?
Your final point is a fine example of ignorance of the law though (please don't think I am having a go at you, I am not). Indeed if the owner of that bank note could prove that you took his property and did not notify the police of your find (your legal duty),the INDEED you COULD be charged with theft by finding.
The crux of this is simple, and here it rests in law.
Are you the owner?
if the answer is no, then you have no right to use said item. Using someones image is no different to riding their bike, driving their car, eating someone lunch that was left on a park bench in error.
One must ASSUME there is an owner and it is our legal obligation to track the owner. In the case of the bank note your legal duty is to deliver it to the police who will receipt it, and if it is not claimed within a set amount of time, ownership defaults to the possessor IE. the finder, as the police were simply holding it on your and the owners behalf.
Imagine someone left their bag on a bench, you picked it up, decided you would keep it. Their keys were inside. A month later they saw their bag, approached you, called the police and you were arrested. They COULD claim the cost of replacing the door locks because you STOLE their property. Just because something is left lying about, doesn't mean it does not have an owner.