Right, but since all this data is available, they can trace it (remember, in your scenario, that they also have internal ids for transactions and tie them to users, and that is how it is resolvable). Surely they should charge a retrieval fee and return the money.
That may be true that they could probably trace it, but how would you propose "returning the money" when it's already sitting in another person's account?
If it's in their MtGox account, there is no problem. If their balance is not sufficient for the account to remain solvent, it should still be put as a liability on their account.
There is no simple way Gox can make this work.
If that were true, they should make it a lot harder to shoot yourself in the foot. Like a real warning!
Although, I bet it is surprisingly easy to figure out what's where from that data (just a graph traversal).