Why does it concern atheists so much?

Because such experience a) uncommon, b) utterly subjective & c) ultimately perverse in that it leads the 'believer' to a mystical sense of 'righteousness' entirely incompatible with the entirely wonderful post-religious reality of human life in the early 21st Century which is a) common to all, b) entirely objective, and c) the natural result of 50,000 of human struggle, striving, wonder, learning, enquiry, discovery and enlightenment.