I strongly agree that their goal should be to remove both the mum and the child together. But in cases where the mum chooses not to leave, then the child shoudlnt have to remain there and be a witness to that behaviour.
I'm sorry you did not get the support you needed. Comming from a professional stand point that is unacceptable.
I'm in Ontario, so I dont know about the states CPS protocal. Just what we do up here. And that was a new mandate in the late 1990's to remove children from abusive relationships. It would shock you to know how many women stay in abusive relationships even when their kids have been removed. So the best thing to do is to remove both mum and child. But when the mum is not willing, the child needs to be protected.
I myself was in an abusive relationship. I have used CAS and have had ot used against me. It is not perfect. Far from it.