Journalistic objectivity?

I started browsing an advance copy of a new book - Tap Dancing to Work: Warren Buffett on Practically Everything, 1966-2012 - that collects all of the coverage of Warren Buffett in Fortune magazine over the past six decades. It was put together by Carol Loomis, whose first paragraph in the book's preface struck me as odd. Here it is:

Because I have long been the chief writer about Warren Buffett at Fortune, which for decades has covered him more closely than any other business publication, I have often been asked whether I'm not going to branch out and write a Buffett biography. I have always said no, sure beyond a doubt that a writer who is a good friend of the subject does not make a good biographer. And I have indeed been a close friend of Warren's for more than forty years, a shareholder in his company, Berkshire Hathaway, for almost that long, and the pro bono editor of his annual letter to shareholders for thirty-five. All of those facts can be accommodated in my Fortune articles about Buffett, simply by my informing the reader that they exist. But they are not a firm base for a wide-ranging personal and professional biography, in which there should be considerable distance between writer and subject. Its absence in this case settled the question.

I don't quite follow the logic here. Being a close friend of Buffett precludes Loomis from writing a bio about him, but not writing about him in Fortune?

The other interesting thing is how adroitly Buffett and a few other businesspeople - Jack Welch and the late Steve Jobs come to mind - handle the press by picking out a favored few, who they then treat as friends and grant insider access. I wonder how much that has contributed to their reputations and whether there's a lesson in that for CEOs?