I'm just going to assume you've spent very little time with people who have adopted children out of situations like that if you think long term trauma or behavioral issues are something you can't assume or are uncommon. No one is justifying abuse because of this. But it's just sort of basic child development. Being abused while the brain is developing does not typically lead to healthy and resilient children with no long term problems.
I think there is a real disconnect in your comment to readily admit the children must have been through "a high level of abuse and or neglect" but that no one has the right to assume it lead to any long term issues with the children. Sorry, but children are NOT that resilient. And all the research out there substantiates this. Why would anyone attempt to downplay the long term effects of childhood neglect and abuse?
We can't simultaneously want children removed from abusive situations and then try and deny the long term effects of that abuse on the children. It doesn't absolve anyone who further mistreats them by admitting the long term difficulties they face! But it's not fair to anyone to deny the problems. It's especially not fair to the children. It's only acknowledgement of the issues and needs that helps insure that these children and their families get the support they need. And, yes, the families need support. The act of adoption doesn't actually automatically qualify any adult to deal with the psychological needs of children who are abused.
Families who adopt and the children they adopt need MORE long term support and care. And they need it in large part due to the very high rate of long term issues due to any previous abuse.
https://www.ncbi.nlm.nih.gov/books/NBK195987/
https://www.childwelfare.gov/pubpdfs/long_term_consequences.pdf