I think it is a fine and dangerous line to cross for a writer. If I were to write about the experiences of a gay Nigerian farmer, I am being highly disrespectful as a writer and possibly instilling my white privileged bias into that story and not realize it.
Now if I interview people and empathetically connect with real people and experiences and use this information to create a fiction then I believe I am giving due respect, especially if I give credit.
Writers MUST do this otherwise all fiction will be monoculture because we can only truly understand our own unique set of cultures and life experiences. Having a diverse range of characters in a story makes for a better story.
I definitely agree that Shriver is being disrespectful though. Just saying its fiction doesnt give one a pass from criticism.