You guys seem to be under the impression that this "could" somehow be optional - perhaps I was not clear enough in my first response. It wasn't an "opinion" as to why it was done that way. I have tried to explain this in more detail on the
Count Pages plugin thread here where the same question came up.
The only way that I am aware of (and Kovid/chaley etc can correct me if wrong) right now that we can safely guarantee that it could silently update your books in the database is if Calibre stops you from doing anything from the point you choose to start downloading to the point it finishes (rather than a background job). Guess what - that is what Calibre used to do, and users (quite rightly) complained that it was a horrible user experience.