As a side note. Terry made an great point
What parent actually believes Jehovah will forever condemn a child because the parent has agreed to medical treatment? Answer: NONE
Why then do JW parents let their children die? Is it about themselves? Are they afraid it is them that will die as a result, or is it more the fear of being disfellowshipped.
Although I've never been a parent, I don't know if I agree with this. I think some JW parents view with disgust the idea of a stranger's blood entering their child's body, as JWs feel in general towards blood, but with the added protectiveness that comes from the parent-child relationship. I could also see the parents worrying that somehow the child will be spiritually impaired for the rest of their life (getting less holy spirit, maybe) as a result of the transfusion. In other words, it's the basic fear of being tainted by taking in that blood.
Besides that, your question presupposes that JWs are thinking rationally. People wouldn't be calling it a cult if its members thought rationally, would they?