Someone who sounds like he knows his stuff drops me an e-mail and explains it thusly:
A key difference between the two cases is the type of documents left behind.
Mr. Bernier left NATO documents un-secure. This meant that he was no longer acceptable on the international stage, because the international stage has standards for proper behaviour. He is known to be untrustworthy with their secrets. Harper wants to be on the international stage... Canada is Back and all that. Mr. Bernier has to pay for messing that up.
Ms. Raitt only left DOMESTIC documents (apparently), and as such the international community is not going to reject her. Well, they might laugh at her, but she is not going to be visibly rejected in international meetings. She is only a fool with our secrets. They will happily take as many of them as they can get. A little domestic faux pas is not likely to affect the Canada is Back meme.
So Bernier is an unacceptable international risk; Raitt is only a harmless domestic imbecile. Sure, I can live with that.