I don't think it is as simple as that. The girl i mentioned only works part time, but still manages to spend very little time interacting with her son, what with after school care, clubs and so on.
I think it must be to do with quality of care. There must have been many unhappy SAHM who were disadvantaging their children if they were depressed and unresponsive.
As Oliver James says, it's not as simple as be at SAHM. If work fulfills you, or you need to work for financial reasons, then you are just as able to be a loving, responsive mother when home. The issue seems to be the childcare until 5. But what happens when the school class ratio is 30:1? Is it suddenly catastrophic for young kids to have less full on attention? No.
I sent my ds to nursery one day at week when he was one. I had to attend hospital and had no one to help out, family too far away. He hated it, cried every week. But I don't automatically assume the nursery was a bad place or he was neglected. I know that he is an only child, and prior to this, he was by my side 24/7. The separation was a shock to his system. But somethings have to be done, and he would need to learn to separate eventually, to adjust to school and so on. My mother was hyper critical of me, but the truth is, she was a depressed SAHM who was emotionally absent for us four a lot of the time.That instilled in me a fear of quiet suburbia.
There has to be a middle ground. 