I wonder if myths like this explain some fraction of a country's economic growth. I suspect that in Europe, this kind of myth would end with the perpetrator being executed for lying and committing heresy; even the Geek myths wouldn't have a god show up to make good on someone else's bogus promises.
Eliezer Yudkowsky also suggested this, at least regarding this African myth: http://robinhanson.typepad.com/overcomingbias/2009/02/an-afr... . I'm a little unsure about how much is the myths and how much is just survivorship bias. A lot of old gruesome European fairy tales stopped getting told after the rise of liberalism and the free market, so it may just be that the myths reflect the current status of society.
"Bias" is not the only kind of survivorship out there. If these myths keep people alive, the cultures that follow them will stay alive. I suspect that the story mentioned above is part of a larger group of stories with healthier morals. The Grimm's fairy tales you mentioned are similar: some of the older, gruesome ones have morals like "Don't travel alone if you're a young woman, or you'll die," or "Don't convince people they're in danger when they're not, or everyone you love will be killed." Those are healthy myths; a village that believed in them would probably be safer than a village that didn't. And once those myths were no longer necessary, they'd disappear, along with some of the more noxious stories.