Forget about ethics for a while. Who should have ownership versus who does have the ownership differs. The protocol listens to the latter. And since ownership is defined by the protocol, whoever has the keys is the owner.
It's almost as if Bitcoin is designed to be electronic cash 
Old fashioned paper banknotes are exactly the same: you may have found it, and you may not be the owner based on ethics, but when you spend it, none of that matters to the receiver.I was stuck on: technically, you can spend the coins when you've got the keys, and technically, you can drive the car when you have its keys.
It's just that in the former, the protocol sees you as the owner, while in the latter case, the protocol doesn't - no matter how many miles you put on that car.