Why does InDesign insist on saving changes to book files?

Why does InDesign insist on saving change to book files when I've done nothing to the book file itself?

This behavior has been going on ever since I can remember, and still happens in 15.0, the very latest version.

I open a book file, then I double-click a document file inside the book panel. 

I work on the document file, save my work, and then close the document file.

I've done nothing to the book file itself, yet if I try to quit InDesign, it always presents a dialog box asking if I want to save changes to the book file.

Huh? I didn't change the book file, so why does InDesign ask  me to save "changes?"

To make matters worse, if I click Save, it doesn't just save the file.

Nope. InDesign then presents a Save As dialog box, defaulting to the same book file name and location. 

So then I click Save again. But of couse, that's not going to work, because the file by that name already exists in that location, so I must click Replace.

This is not typical Mac application behavior and it's infuriating because it prevents me from easily quitting the app or shutting down my Mac until I've dealt with "saving" or ignorning changes to each and every book file opened during the work session.

Am I doing something wrong? Is there a way to prevent InDesign from insisting I save unchanged book files?