The world doesn't operate on prevention, because prevention doesn't work.
They were able to reverse transactions and roll the site back to the way it was before the invalid order. They also had mechanisms in place to reduce the amount of irreversible damage that could happen before the attack was noticed and stopped.
Those sound like pretty damn good safeguards to me.
Tight security is a pretty important form of prevention. Mt Gox can rollback transactions as much as they like. The few members of the general public who follow bitcoin got the message that 'bitcoin got hacked', regardless of the real details.
The world does operate on prevention. That's why balconies have railings, dangerous areas are often fenced off and industrial machinery has physical and electronic barriers to prevent accidents.