When you adopt a child, you know little to nothing about them, unless it happens to be an adoption within a family or something similar. You know only what the adopting agency wants you to know.
Let me put it this way. If an agency had to write a "listing" for my seven year old, it would go something along the lines of:
7 year old white male, raised in a large two-parent household. Exhibits mild traits of autism spectrum disorder, physically healthy. He is in second grade, where he is excelling in the areas of reading and writing. He would do best in a two parent household, where nurturing one on one time can be a priority.
That really doesn't sound that bad.
However, my brutally honest listing for the same child would read something like:
7 year old male, raised in a two parent household. Is diagnosed with pediatric schizophrenia. He poses a severe risk to himself and others if not properly medicated. Even when properly medicated, he must be directly observed every waking moment, including while using the bathroom. Strong signs of physical aggression, coupled with his physical strength, make it necessary to recommend that he be placed in a home with at least two physically strong adults, as restraining him is necessary daily. he is excelling in school, but will not even attempt subjects that don't interest him, therefore, constant monitoring and academic services are necessary. Constant attempts at manipulation and control, while refusing to accept responsibility for his own actions makes it imperative that the entire family have access to counseling services and respite care. Death threats are common, so is self injury. One parent will need to be available at all times, even if he attends school, so that he can be picked up from school, taken for crisis mental health services and emergency treatment for self-injury. It is recommended that pets, prized possessions, food, and any potential weapon (read: anything he can lift) be locked up securely.
That's pretty much the difference. You take on a toddler and within a year, it is clear that the child has no way of giving or receiving love, so traditional discipline is useless. Beating the child might work, however it is illegal, and will teach them nothing, however, by the time of the adoption, is it possible that the child was so badly beaten that abuse is all they respond to. Agencies are supposed to disclose everything they know to the adoptive parents in regards to the child's health and history. Reputable agencies do. Non-reputable agencies don't, and even when info is given out, sometimes it takes years for the effects of the hell the kid lived through to start showing.
This is a well known practice where I come from in the low income world. It's called "Beating the system". It's what abusive or poverty stricken parents do when CPS knocks on the door. They write out a guardianship paper, have it notarized, and sign over the child to someone close to them. As soon as the state stops looking into the case, the child is returned to their home. It is perfectly legal, and honestly, it has the chance to save lives. It might end a few too, and put children in worse situations, but often this is the only chance that child is going to have, or they might be beaten to death or just thrown out on the streets anyway.
There is no easy answer, that's for sure.
(Additional note: No, I have never tried to give away my son, the post above is written simply for explanation sake. I love him, he is mine, and I'm not letting him go anywhere.)