Crazy vegans and social evolution

This horrifying article appeared in today's AJC

Vegan parents guilty in infant murder
6-week-old died of starvation after being fed diet of soy milk, apple juice
The parents of a baby that died of starvation after being fed a vegan diet have been found guilty of malice murder, felony murder and first degree cruelty to children.
...
Prosecutors said it was a chilling case of murder by starvation, a painful and prolonged death. Attorneys representing Sanders and Thomas told jurors the first-time parents did the best they could while adhering to their vegan lifestyle. Vegans typically live free of animal products.

It's troubling in many ways; it raises the question of do we need an official (i.e. government) of raising children (no), and how could these two be so stupid as to not notice that their baby was shrinking?

The truly rare thing is how did these two avoid the self-appointed legions of women who see an infant as an invitation to ask the parents questions on every conceivable subject? It's not like you have to seek out child-rearing advice when it comes flying out of the woodwork in public places. I imagine it's decent advice too, just repetitive.

Perhaps it's an evolved behavior. Post-partum depression being common a society with an army of cooing watchdogs is the first line of defense against neglect or abuse.