If it was one of Chretien's lackeys caught doing this BS on the public dime, I doubt very much that any one of us would cut them any slack for it, so why should we do so when it happens to be someone that we voted for? The overwhelming reason that most of us voted for Harper was because we expect him to be responsible with the public purse, and everytime that nonsense like this comes to light, the more it reinforces the perception that the only difference between political parties is the colour of the posters they stand in front of when they lie to us.
Mind you, the more often expressed view among CPoC supporters is that MacKay is entitled to his entitlements:
As a Minister of the Crown, private or commercial transportation should be used whenever reasonable to do so. That being said, MacKay's record as a Minister has earned him the right for us to cut him a little slack on this one even if it may have been inappropriate.