Great article explaining some profound details about ownership:
http://www.coindesk.com/bitcoin-ownership-impact-fungibility/When you aquire a stolen article from a thief, you might be responsible, and have to give the article back to the rightful owner.
Not so with money, when you sell a can of beer to a known thief, nobody can claim the money from you. The reasoning behind it is to secure the fungibility of the money. When you receive money in a transaction, you should not worry about the source of that money. Exactly what we need for bitcoin, for the same reason.
Come to think of it, this is not so with the banks, they worry a lot where the money comes from, and I have to declare by which acceptable means I have come to own them. Are deposit money not fungible? Strange world, indeed.
Nobody can claim the specific piece of paper money from you, solely on the basis that it was the very same piece of paper money that was stolen.
They (and others) can still make all sorts of other claims, for money or otherwise, like if you had reasonable cause to believe the paper money was proceeds from criminal activity, and accepted it anyway (see Shrem). The act of spending such knowingly dirty money to make it fungible, is called money laundering, I thought...