While I understand what you are saying, I believe that in this case there might be more concern about a pedo who hides a child away for years and subjects them to horrible abuse - abuse that may even be live streamed on the dark web - than there is concern about a network of caring people who are trying to keep children safe from alleged abuse by a parent.
The children in the 2nd scenario are kept safe and well cared for, the children in the 1st scenario are obviously not.
Judging by the way the foster parents broke down at the relevant point in their published police interview, I think they are very afraid that William was subjected to some horrible abuse after he was abducted.