I think Cameron is fearful of appearing weak.He confuses stubbornness with strength. He spent far too long supporting Andy Coulson, for instance.
Andrew Mitchell has become a lame duck, it seems to me - He even had to stay away from his own party conference, right on his doorstep, in order to avoid overshadowing the conference with fallout from his ridiculous elitist, entitled comments.From a practical point of view, I find it difficult to understand how the Chief Whip can do his job- which is to elicit obedience to the party line through fear or inducement - if he fails to command the respect of his own party, and thats what seems to have happened.
And his financial support of the Rwandan regime - dedicated as it is to fighting a guerilla war against the DRC - based upon a personal relationship with the president and against the advice of his own civil servants - seems based upon arrogance more than anything else. All in all, I think Cameron will come to regret leaving him in post.