As far as I understand, that's because older systems don't use a digital image sensor to scan these, but rather a rotating laser beam and a simple photo diode registering the variations in brightness caused by varying reflections of the laser beam by the white and black parts of the barcode.
A camera-based scanner doesn't care where the illumination of the barcode is coming from (i.e. ambient light, its own LED illuminating a piece of paper, or an active backlit screen); that laser-based system is purely based on its own reflected light, though, and won't work with actively backlit screens at all. (I wonder if it works with e-ink or passive LCD displays!)
A camera-based scanner doesn't care where the illumination of the barcode is coming from (i.e. ambient light, its own LED illuminating a piece of paper, or an active backlit screen); that laser-based system is purely based on its own reflected light, though, and won't work with actively backlit screens at all. (I wonder if it works with e-ink or passive LCD displays!)