False. Indeed its entirely the opposite. US law is very clear on the ownership of found/abandoned assets. Essentially, to put it very simply "finders, keepers".
Only if you cannot determine the lawful owner.
Finders keepers law is a common law rule that allows a person who finds lost property to keep it, unless the original owner claims it.
But when I said you can't do anything with it I meant if you plan to spend the full million you're going to get burned. Either you have to declare it and pay taxes or if the owner is known, return it. That's why I stated in the Bitcoin example,
"...if you somehow found an unclaimed computer in the trash with no personal info". But I believe, if the username on the computer is public and there are other personal documents identifying a person, it is likely the computer is theirs. I guess an argument could be made that they did not want the computer anymore - similar to what happened with Kevin Costner and his hat in Dances with Wolves. But the ex-owner could claim they lost their computer or it was stolen. If they have a police report then that computer is still their property and you can't keep the Bitcoin. Plus they might have receipts indicating when and how they acquired their Bitcoin.