At the risk of being slightly off-topic, and assuming you don't mind the bump: this is an interesting observation! I'd say we can even stretch it a bit further, and make "taint" disappear exactly the way it does in fiat money: if a criminal has gained a $100 bill by criminal means, that money becomes clean the moment he spends it in a restaurant for a meal. Nobody would blame the restaurant owner as long as he doesn't know he's doing business with a criminal.
Back to the above: by this observation, the "dirty" Bitcoin becomes "clean" the moment it hits an exchange. If we extend that a bit, I'd argue it becomes clean the moment it's received by someone who has nothing to do with any crime. That's how fungibility works, and that's how it should work in Bitcoin.