I still think the best thing for OP to do is simply post the address so the true owner can try to find him. For all we know they are hunting frantically for any mention of this address online.
If if this would be true in the US there is always two sides in law. The law theory and the law in everyday live. If the law in your everyday live is the same as the law in theory, depends mostly if the law in theory can be enforced. This would be the problem here, since bitcoin is anonymous or at least pseudonymous. So in this case there may be the theoretical law in the US that you can not keep the funds, but in practice there would be nothing that would you stop from doing so.