John Irving was never a member of the SS or a winner of the Nobel Prize, so while there seems to be less at stake for him, writing is Irving's life. To deny Grass would mean denying his own life, unblemished as it seems to be. However, there is something deeper than loyalty and career at work here. Irving, an American writer, feels obscurely that just being a writer may involve an insoluble moral compromise. Irving does not say this explicitly, but his apology is also shot through with guilt. We don't know -- or at least I don't -- what weighs on his conscience, but I can tell from my own writerly experience that some of the best images I came up with were conceived at the expense of some truth, partially or in whole.
Not all truths are the same, of course. Being in the SS is not the same as saying that my grandmother was a baroness who raised swans, instead of a peasant who raised pigs. Changing my lineage for the sake of a better understanding of who I am (or for the better amusement of my readers) is not the same as killing for Hitler. The transformation of personal truths for literary reason cannot be explained away by literary conventions, either. You can call it memoir, fiction, comedy, tragedy or doggerel, but the insistent fact of hiding or obscuring the truth remains. One can argue further that writing is in itself a mistranslation if not a downright betrayal of experience, but just how shabby a line of thought that is was amply demonstrated by the unmasking of Paul de Man or Mircea Eliade, thinker-tricksters who sought to hide their fascist pasts by clever inquiries into the unreliability of language.
John Irving created many fine symbolic images in his lively fiction, but their vogue has faded in time. In fact, Gunther Grass' own poetic images had pretty much faded until the recent revelations. In a curious, but not atypical way, the use of truth by both Grass and Irving is an attempt to revive their fictions. The gambit doesn't work, not only because the truths are of such different gravity, but because when truth is revealed, fiction wilts away like an old onion. Where once there were tears, now there is only compost.