It’s just incredible the injustice that can be done in the name of protecting children. I really do wonder if it’s cultural or some kind of innate psychological irrarionality that seems stronger in some than others. I love kids and care deeply about their welfare, but people sometimes try to make me feel bad or that I’m the weird one for being able to think (I believe) fairly rationally about the risks and dangers that they face, instead of massively over-exaggerating!
Or of course the opposite, keeping an appropriate eye on relations and acquaintances when people assume they’re totally safe but it’s actually somebody with that level of relation who’s likely to be a danger than a stranger.
This is just speculation, but I bet those groups (or their members) aren't always calmly and coolly trying to find the best policies protect the welfare of children. Instead they feel themselves on a kind of righteous moral crusade, and what's more heroic than swooping in to take the child away from the clutches of the villain? The feelings of heroism could obscure understanding the harm the "heroic act" could cause.
For the people in child welfare organizations, for social workers, for doctors, for police, for judges to change their mind about current and future decisions requires them to change their mind about past decisions. The necessary implication is that many of the people they have persecuted in the past were, in fact, innocent. It requires them to admit that they personally have likely caused untold suffering to parents, caretakers, and children.
This is hard for anyone; but if you've lived your life trying to be the hero, feeling good about swooping in and rescuing children from the clutches of evil villains, how can you face the fact that you are the evil villain in so many children's stories?
You might call this the Paradox of Judgment: If you don't say that something is that bad, then lots of people don't think it's a big deal and don't do anything about it. But if you do say that something is really bad, then there develop all these pathologies of denialism around it.
People like me who challenge the science behind the diagnoses of SBS face an absolutely unprecedented and unreasonable pushback, like I've never seen in any other area. Basically everyone who has worked on this side has faced threats, insults, personal attacks, cancellations, boycotts, and so on. The "cognitive bias" you mention (does it have a name? perhaps cognitive dissonance?) is a likely reason for this amount of antagonism.
This is why Max Planch (German physicist) has quipped that science advances one funeral at a time.