@Bella, I dont think of it as child cruellty at all
He is loved, he was safe in the bedroom, within 3 nights he was sleeping through the nights. If he was still screaming 6 months later i'd be agreeing with you. But 3 nights? Nah. He was probably crying for 3 hours a night prior to having his door locked.
That's not really how it works. If your husband locked you in your room and you "only" cried for 3 nights and he was lovely the rest of the time is it not domestic abuse?
Of course he only cried for 3 nights, he's an intelligent human being. The first night, as you will have read, he screamed for 3 solid hours. He actually fell asleep curled up behind the bedroom door... The second night will hav been less of a shock but still very upsetting and the third night he's getting more used to it. After that, he knows the score. No point bothering, it feels worse to cry for so long than just giving up and getting on with it, no matter how you feel inside. It would be the same for anyone.
The thought of that poor child locked in his room screaming so hard that he went hoarse for a full day and curling up behind the door as he was desperately trying to get out and be heard until he was so exhausted that he fell asleep where he was knocks me sick.
To the people who repeatedly say children need to learn and assorted fun stories like "Zomg I once knew a 50 year old man who still co-slept with his mum! Co-sleeping is to blames!!!" ... Why do they need to learn? On average most children, left to their own devices, start to sleep along around 8 or 9. What is wrong with a 12 year old getting in with their parents? 12 is still so young in the scheme of things, 3 is still just a baby! Some children don't like to co-sleep and that's fine, it would be madness to force it, but if it's just "not for me" then meh... You're the one who needs to learn, to be frank.
I think it's just to close to home for most people as it's not that far from CC/CIO and not all that different to leaving them in a barred cot or playpen or behind a safety gate. I imagine that would be an uncomfortable feeling.