I agree that this is hideous and cruel but before berating the parents I think you need to inform yourself about their backgrounds. Slim is so right in that the roots of this behaviour are poverty and ignorance and, I think, powerless-ness. The parents are as much victims as the children.
Until you've lived, like I do, in Nigeria, it's hard to imagine a society where people can actually believe such claptrap but when you have nothing else in which to inform what happens in your life, you grasp at straws. 'Common sense' to such people dictates that if you are sick, it's caused by a witch, because you've never heard of germs or bacteria or parasites.
Life is also cheap here. Most people believe in some sort of afterlife so dying isn't such a big deal. My dh's work involves him in H&S issues and it's a constant uphill battle to get anyone to take the messages on board because of this.
The influence of religion is massive. There's a huge new church just down the road from us, (with under-church parking )whose congregation is by invitation only. I find it hard to stomach such churches existing when people right outside have no shelter, no clothes and no hope and it's not surprising that, again as Slim says, they cope by turning to such fantasies as witchcraft.