Overstock isn't accepting BTC, they are accepting USD from Coinbase, who in turn accept BTC. Coinbase bears the security risk.
as soon as there's enough money, well funded organizations will start attacking, and robbing a bitcoin wallet is probably less risky than a bank...
They are probably one of the groups with the best BTC storage security (while maintaining usability). So yes, if they WERE to be robbed, then that would basically mean that no one is able to properly secure BTC.