And no one's pretending literary biography is equivalent to literary criticism.
As a fan of literature I like reading analysis of literature, and also enjoy finding out about the lives of my favorite writers. Why is that so wrong? How is my curiosity about or knowledge of their lives harming them?
The other point is that certain details seem better unknown. I mean, why should we even know that he burnt any part of her diary? To me, that sort of thing crosses over into mere voyeurism. He may be the very one who told the story, but I've never been a fan of the confessional style of literature, which unfortunately seems to be almost the only thing left these days.
Of course, if the criticism is baseless and it causes harm to the target of the criticism during their life, that's wrong.
But that's very different from merely trying to find out more about a person's life -- especially once they're dead and can no longer be harmed.
"The other point is that certain details seem better unknown. I mean, why should we even know that he burnt any part of her diary?"
To satisfy one's curiosity seems like a good enough reason to me. Curiosity is a big part of why I read literature in general. I want to find out about how people who are very different from me or who lived in very different circumstances or times lived. It's the next best thing to being able to live multiple lives myself. I say, why not?