I'm curious about something. A hypothetical situation. If Ben was asleep while the coins were being sent to his wallet, which was stored in an encrypted file on his computer, and when he woke up, he couldn't remember the keys for decrypting that wallet file, would he still owe the sender the bitcoins?
Are you saying he could say he didnt get the money? Not in the bitcoin social contract. He couldnt complain about that in the normal course of business(as in 'hey I didnt get the bitcoins I paid for from buying bitcoins because I cant remember the keys to my wallet') and neither can he complain about it now. It all goes to the social contract that he voluntarily accepted. Had he notified the owner before, and said he didnt have his keys, then that would be on the owner. Can you imagine the owner saying he didnt get bitcoins from BenDavis on a sale because the owner forgot his keys?
The fact that he took the bitcoins and acknowledges their receipt and professes his right to keep them, he's a little late for that.