... because any wallet downstream of the tainted wallet in the transaction graph could be considered tainted as well. This would be a highly effective attack on

... because any wallet downstream of the tainted wallet in the transaction graph could be considered tainted as well. This would be a highly effective attack on Bitcoin as an illegal payment method - the onus would be on every Bitcoin user to ensure that they have provably done everything they could to avoid obtaining "illegal bitcoins". Therefore, the person in Russia in your example would have an interest in not dealing with tainted coins, because they couldn't be sold for the same price anymore. It would basically split Bitcoin into white (provably legit), grey (unsure) and black (illegal) coins. Yes, this would probably also destroy Bitcoin, but that's a mechanism by which states could obtain extraterritorial power over what people do with Bitcoin.