I don't get your fixation on this. There was an ongoing court case that could not be reported on (the Daniel Morgan murder, Rees and Fillery in the dock). It's exactly the kind of information you'd expect an editor to pass onto a senior politician who is about to do something so incredibly fucking stupid. I'm actually quite surprised that no other paper did, in private. Whatever their own dodgy dealings, they must have known this would blow up in his face.
Plus, Rusbridger had been chasing this story for years. Given that Cameron would ignore him on principle, getting it on record that he had been warned was arguably a bit of a master stroke.