Yes, some other sites have mentioned that if an unclassified computer gets infected with classified data, it is a big problem because then the whole hard drive has to be shredded, as a standard security procedure. Imagine that happening to the mail server because somebody sent an e-mail about a cable he just read...
So, because nobody is willing to either declassify the lot, or create an exception to the security procedure, official unclassified computers must be kept clean by staying away from the bad stuff... This is actually a fairly valid explanation for a bureaucracy with many layers of rules.
However, this does not explain why university students have been warned to not talk of the cables on social networks, in case they ever want to work for the government. This may be paranoia, but it is fairly bone-chilling.