This type of incident is a key reason the US Treasury's Financial Crimes Enforcement Network ("FinCEN") issued sensible guidance requiring those who offer wallet (bitcoin transfers between persons and/or merchants) and currency conversion services to be regulated as money transmitters. While money transmitter regulation isn't perfect and is expensive for providers, it can provide some consumer protections in these scenarios.

Edit: Forgot to link to their guidance

http://fincen.gov/statutes_regs/guidance/html/FIN-2013-G001....