The truth is often in the mushy middle. Science can take a decade or more to catch up with what was common gossip among parents.
Or not.
The dynamic that drives this is that when it's your kid, a scientist's answer that "we really don't know what causes (for example) autism" is just emotionally unacceptable.
So parents grasp at straws and try anything, no matter how whacked.
One can sympathize with them while still rejecting their views as public health policy.