[vsync]

Paper doesn't shine into your eyes.

We also try to avoid staring into the sun, for the same reason.

[mrob]

What objectively measurable difference is there between reflected and transmitted light?

Polarization might be different, but that's something that's only barely perceptible to humans. I don't think it's plausible that it could affect legibility or eye strain. ( https://en.wikipedia.org/wiki/Haidinger%27s_brush )

[s_kilk]

Energy transfer.

There's a big difference between absorbing reflected ambient light and having photons pumped into your retina.

[aetherson]

What? No. You obviously receive way more energy via sunlight reflected off the page of a book than you do via your LCD screen. That's WHY you can't read your LCD screens in direct sunlight.

[Sami_Lehtinen]

If the display shines into your eyes, you've mistuned it. Displays white should be as bright as paper sheet you're holding next to it. Fix the settings now. It'll be good for you. Low contrast displays and screens are bad. That's why contrast optimization is so important, like on Kindle. Every new model got higher contrast than previous ones.