Children of parents who are severely bipolar are in danger because this is a serious mental illness that causes erratic behavior. The child is likely to be either abused or neglected during an episode or at least will be forced to act as a grown up too early in life in order to survive physically or emotionally.
I believe that my mother's mother may have been bipolar. Her childhood was extremely messed up as a result. While she has managed to recover fairly well from that experience, many of her siblings are still feeling the effects of that severely. I do believe that that all of them would have been better off somewhere else.
I don't think that it is acceptable to remove a child from a parent simply because the parent has been diagnosed as having bipolar disorder. Bipolar disorder does not mean that someone is inclined to be an abusive parent. What if there are two parents in the household, and one of the parents doesn't have bipolar. In such a situation you couldn't justify removing a child.
A parent with bipolar disorder, who does not take medication, is not a direct threat to their children. It is also not a directly correlated with child abuse either. For these reason, it is unreasonable to assume that children should be taken away from bipolar parents simply because the person doesn't take medication. Bipolar is difficult to diagnosis and in some cases it is diagnosed incorrectly. Tearing families a part should not be considered okay.