I don't think it's inconceivable. I remember hearing that many users of the GlowLight-equipped Nook kept the light on since it dramatically increased the perceived whiteness of the page.
On Amazon's Paperwhite product page, there's no claim anywhere that the light can be turned off. If anything, careful wording seems to avoid stating that it can be disabled: 'You can adjust the brightness of Kindle Paperwhite across a wide spectrum - from low light to full brightness - so you can read comfortably in any setting.'
Edit: I'm also not seeing an Off switch
here.