I don't think there's any indication that they actually thought destroying the hard drives would stop anything; seems more likely that:
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...