I once
read in a medial journal, althought I cannot remember which one, that when a person begins to
experience death the
receptors in the
eyes of the individual begin to loose blood and start to
die themselves.
This could easily cause random "noise" signals to be generated by the eyes and fed into the visual cortex of the human brain and be percieved as white light - hence - the tunnel and the white light near death.
Seeing the white light could be the brains way of calming itself (your brain) down right at the verge of death by making your way into death a little more relaxing.
Wether the article had any merit or not, it's plausable and a damm good explanation of where the white light comes from in near death. The viewers do not actually die but their brains did see the white light at some point and that is what they remember.