Alleged victim sent BTC to MtGox, but used a MtGox address that was not tied their account as a "Funding Address".
So, your title is wrong and misleading.
The alleged "victim" made a donation to Mtgox and now is crying for some bitcoins.
Mtgox is not a charity. They have all rights to keep those btc.
If I sent you some btc, that would be nice if you send it back to me after I contact you. But that's not compulsory.
And you keeping the btc, will not make you a thief. You did not steal anything, if I sent you the bitcoins in the first place.
You don't have to give it back to me if you don't want to.
Case solved.
Sorry, but that does not work for a "real world" registered company only for a shady private person (and even here it depends on the laws of the land).
ding ding ding!
In fact, if you compare it to real world currency...
At least in the quite a few parts of the US:
You write down the wrong address on an envelope and send me 500 dollars cash. You have some way of proving this (say for argument you took video of the cash and putting it in the envelope and sealing it, showing the address, and following all the way until you dropped it in the mailbox at the post office)
You contact me and explain you sent the money incorrectly, and I admit and acknowledge that I did receive the money, but claim I can't get it out of the mailbox - that it's a send only mailbox and I can't receive with it, but then someone has proof that I took the money from that mailbox and deposited it in my account (again let's say for the sake of argument videotape)
You could sue me for that money and in quite a few courts you would win, end of story. Just because someone sends you something accidentally does not give you the legal right to keep it in most jurisdictions. Some are exceptions, and some have specific rules for businesses sending merchandise unsolicited that allow the receiver to keep (to prevent businesses from sending unsolicited merchandise and then billing the recipient, and demanding they pay to ship it back if they don't want it)
Gox does most definitely have a moral obligation to return those funds if the address is under their control and they've swept the funds out.
Gox may have a legal obligation as well depending on the various jurisdictions involved and how hard the user chooses to push the issue.
Regardless of any of that, this serves as a warning to most that they can't trust gox to 'do the right thing', but can only trust them to do what they absolutely must or are forced to.