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.
I don't disagree. If someone was to find one of my paper wallets, or stumble across one of my seed phrase back ups, or if I left a hot wallet unlocked in public, then I fully expect that all those coins would be stolen, that there would be nothing I could do about it, and that law enforcement either wouldn't care or would be unable to track the person down.
But that doesn't change that the act of stealing my coins is still morally wrong.