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. ...... But I'd like to think the best course of action would be:
1) Wait for information about why it's shut down
2) Look up who made the arrest (local police, FBI, whatever it might be)
3) Issue some joint user statement about the funds being held captive, petition them to release all funds not connected to suspected accounts
+1