Why do people tell sick jokes about tragedies?

From the ever-astute writers at the BBC, about how the human mind copes with devastating horrors:

http://www.bbc.co.uk/news/magazine-12775389

Tasteless tweets about the Japanese tsunami have landed celebrities in trouble. So what makes people tell sick jokes about terrible disasters?

Have you heard the one about the tsunami that killed thousands of people?
If not, someone might have told you a gag about the threat of nuclear meltdown. Or any other horrific event that happens to make the headlines.
Death, destruction and widespread devastation may be the unfunniest subject matters imaginable. Yet for some people they make up a comedy sub-genre...