If each bitcoin has a ledger (Peer to Peer) and contains all the transaction history since it was mined, then how come it can be stolen? For example, Bitcoins were stolen by hackers from Mt. Gox. If the Bitcoins each have a ledger it would show its owner and the Mt. Gox account. So it would be traceable and hard to trade if stolen. Maybe I am not fully clear, would appreciate clarification - thanks!