The original folk tales are often really extremely dark in content. There are common themes running through them which reflect the fears and life experiences of the people at the time. These stories would, originally, have been part of an oral tradition. Stories would have been retold again and again over the years, sitting around fires on cold, dark winter evenings. Accompanied by drinking more than likely. Professional storytellers would have travelled from village to village peddling their tales. Only much later were these stories written down by such people as the Brothers Grimm.

If you can, and have not already done so, read the originals. They are very far removed from the Disney versions. They are often violent, disturbing, weird and traumatic. In Sherlock it is the tale of Hansel and Gretel that is a point of reference in The Reichenbach Fall. What a jolly little tale that is! Death of mother, evil step-mother, rejection by father and attempted child abandonment in the forest. Return and rejection, further child abandonment in the forest with death by starvation, thirst or from wild animals the aim. Capture by a witch using a gingerbread house as a lure. Boy put in cage to be fattened up for the witch to cook and eat. Girl used as slave labour. Witch pushed into heated over and burned to death. Children find way home, step-mother dead and reunited with father (bit dubious about him though, I would think).

Maybe, if they are ignorant of the original stories this is tapping into the very human enjoyment of the frisson of being scared. Many of them may not know exactly about the stories but they are still part of folklore, in its widest sense.