What happened here was that their hand was forced by the reckless acts of The Guardian's Leigh and Domscheit-Berg. One key reason access to these unredacted cables was so widely distributed is that Leigh -- in his December, 2010, book about the work he did with WikiLeaks -- published the password to these files, which was given to him by Julian Assange to enable his reporting on the cables. Leigh claims -- and there's no reason to doubt him -- that he believed the password was only valid for a few days and would have expired by the time his book was published.
That belief turned out to be false because the files had been disseminated on the BitTorrent file sharing network, with that password embedded in them; Leigh's publication of the WikiLeaks password in his book thus enabled widespread access to the full set of cables. But the key point is this: even if Leigh believed that that particular password would no longer be valid, what possible point is there in publishing to the world the specific password used by WikiLeaks or divulging the types of passwords it uses to safeguard its data? It is reckless for an investigative reporter to gratuitously publish that type of information, and he absolutely deserves a large chunk of the blame for what happened here; read this superb analysis by Nigel Parry to see the full scope of Leigh's culpability.