It's fairly easy to obfuscate the source of coins. There are "laundering" services where you can send a bunch of coins and they will redistribute them randomly

It's fairly easy to obfuscate the source of coins. There are "laundering" services where you can send a bunch of coins and they will redistribute them randomly mixed with others who use the service. Even if you could reliably follow them all, what agency would keep track of which coins are "tainted"? For an amount of money this small, it's just not practical. Even for the many thousands of coins stolen from MtGox, tracking them is not practical.