Also, there was always a possibility that the guys in the guardian slipped somewhere. Didn't have a copy of all the files, some password/key was stored only on that drive, note/edited information that's easy to forget was only on that drive, etc.
Backups are hard - if there was even a slight chance that they destroyed the only copy of something, why not go ahead with it? They're not that stupid.
1) They didn't want anyone else to get access to the material that the Guardian had but would not end up publishing;
2) The rules said that that's what they had to do, so they did it;
3) That's all they could do on "their patch", so CYA;
4) A little intimidation would never go amiss, surely...