This may or may not be true. Many jurisdictions have a minimum level of identity verification which must be applied to all accounts no matter how small the balance/transactions and funds can and will be held indefinitely if the customer does not satisfactorily identify on request. Certain types of transactions trigger ongoing and enhanced due diligence requirements and providers can ask you to re-verify at any time.
Very few jurisdictions in the world entitle anyone to take indefinite possession of somebody else's property. That's because, if anything, whenever there exists funds floating around with nobody's name on it, those jurisdictions will want to "escheat" those funds for themselves! If MtGox is acting on a purported obligation to hold someone's funds "indefinitely" for legal reasons, there's almost certainly a proper procedure that must be followed for turning those funds over to the proper authorities.
Only when MtGox can prove it has done so does it have any excuse to deny a refund.
MtGox is free to refuse to accept anonymous people as its customers, but it may not rightfully confiscate their money for their own benefit. The only reason MtGox really gets away with it is the person has no recourse - they're probably anonymous for a reason. MtGox is exploiting their disadvantage, no differently than a mugger exploiting my solitude in a dark alley as he takes my money. It is not right.
Letting people legally be the judge, jury, and executioner to arbitrarily convert someone else's property on a whim would be a serious moral hazard anywhere, and I'll bet you'll be hard pressed to name a single jurisdiction anywhere in the world that would allow MtGox to just choose to keep the funds "indefinitely", let alone one that has MtGox's offices within its borders.