Even if some people did use it for laundering BTC obtained from illegal sources, I'd say a vast majority of the users are legitimate. I'm not sure exactly what the allegations are, but it sounds like the arrest is due to money laundry. If they indeed had anything at all to do with helping with this, or if they are just arrested because the site itself is used for this purpose nobody knows. But I'd like to think the best course of action would be
"The detainee was held on four counts of running an unlicensed money-transfer operation, conspiracy, money laundering in violation of 17 acts in the U.S. criminal code, and transactions in cash acquired through illicit means." -
http://www.reuters.com/article/us-greece-russia-arrest-bitcoin-idUSKBN1AB27W2) Look up who made the arrest (local police, FBI, whatever it might be)
Local police, acting on a U.S. warrant.