Debating biases aside... An author not interviewing their subject is unprofessional. (edit: http://en.wikipedia.org/wiki/Hearsay )
Reporting on a subject without interviewing them is so common, and so necessary, that journalists have a term to describe it. It's called a "write-around."
Full disclosure: I worked with Brad Stone for two years.
Presumably Mr. Stone had evidence and reason to put Bezos's mind in a certain frame from circumstance, action, or interviews and if he did, he should have laid out that evidence to show us his probably mental state and not simply told us.
Is there a reason for not doing so? I agree it seems really unprofessional.
E.g. this paragraph, in the middle of like stuff, is devastating:
"In light of the focus in many of the reviews here and elsewhere on what the book “reveals” about Jeff’s motives, I will also point out that the passage about what was on his mind when he decided to start Amazon is far from the only place where the book passes off speculation about his thoughts and intentions as fact. “Bezos felt…” “Bezos believed….” “Bezos wanted….” “Bezos fixated…” “Bezos worried….” “Bezos was frustrated…” “Bezos was consumed…” “In the circuitry of Bezos’s brain, something flipped…” When reading phrases like these, which are used in the book routinely, readers should remember that Jeff was never interviewed for this book, and should also take note of how seldom these guesses about his feelings and motives are marked with a footnote indicating there is any other source to substantiate them."
http://jimromenesko.com/2013/08/11/i-interviewed-jeff-bezos-...
If the subject does not want to do an interview, than it is not unprofessional to not have interviewed the person. In fact, I can't imagine why even a dishonest reporter would not interview their subject given the chance, as that reporter is still in control of the transcribed material and can use it dishonestly, even moreso than if they didn't have the interview, because they can claim their biography has extra weight, due to the exclusive interview, even if they then end up twisting to their purposes.
(which is why very famous people often do not talk to reporters in the first place)
The "writearound", the kind of biography in which you do extensive research to fill in the gaps and talk to everyone else you can find, is actually a pretty well-honored form of biography, and one that can be very illuminating.
(I'm not defending the author in the Bezos case as I don't know the particulars. Just disputing your argument that not having a direct interview destroys the legitimacy of a biography)