Soo what happens if you're incredibly lucky and stumble across the private key for an active wallet? If you transfer the funds out, isn't that just theft? Is "guessing" a private key any different from guessing someone's bank details?
The blockchain has no concept of people/entities owning things, in that universe the ownership of an address is simply having its private key.
(Of course in the real meat-world we have courts, non-code-contracts, and rule of law. It would probably be criminal, in the same way finding a weakness in e.g PayPal and transferring peoples money is criminal)