I think I'm uniquely qualified to speak on this subject. My oldest son (and dad of my thus-far only grandchild) is from my wife's first marriage, and I had a low-functioning autistic Foster child for four years.
First off, I never adopted my oldest son, but I love him deeply and have always given him the kind of priority that firstborn normally get. He was so grateful that without telling me, when he turned 19, he changed his last name to my own. I love him dearly...and his son - my grandchild - has been a life-changer. I can't describe how happy I am when I hold him...but other grandparents will understand what I mean.
The point is, blood relation is not a requirement for a parent to love a child as one's own.
We had several medically-fragile Foster children along the way, one of whom was Sammy, a low-functioning autistic. When we let him go, he was 19 y.o. with a mental age of perhaps 18 months. As is the case with many Foster parents, it was time to move on to a new phase in our lives...but let me tell you this: there are VERY few Foster parents who can properly handle medically-fragile children. It's not just that they're children, but their medical and mental challenges, too.
I loved Sammy. Of all our Foster children (one of whom has aged out of the system and is sitting about six feet away as I type this), Sammy is the only one I ever really bonded with. I loved him...but we had to let him go. We could no longer provide what he needed. It wasn't just his medical needs. but raising an autistic child is *very* challenging.
What's more, when one *adopts* a child, one receives FAR less support from the state...and that is CRUCIAL. Foster parents tend to get a LOT more support from the state, but once the child is adopted, all too often it becomes "okay, you're on your own." That's why we never adopted any of the medically-fragile Foster children we had.
So while my heart breaks for the child, I also sympathize with those parents who let that child go. If they made a mistake, it was thinking that they had the training, patience, and *especially* the marital teamwork needed to care for such a child. Unlike almost all those who hate them, I do have a clue as to the challenge they faced.
They tried and failed...but if they're guilty of anything, it's not malice or greed, but of ignorance and overconfidence.