I had an "update available" prompt a few days ago with options for "Now" and "Later". I tapped Later and it asked for my passcode "so that it can install it overnight" (or words to that effect). I had to tap a comparatively small "do not install" option.
I suspect that people aren't reading properly and are scheduling the update instead of cancelling it.
And that Apple are deliberately obfuscating this option to opt-out.