Postpartum depression IS a real disorder... that's probably why it is diagnosable by psychologists.
I've been diagnosed with depression and was at high risk for postpartum. Thankfully, I resorted to treatment during my pregnancy.
Many people think of depression itself as an "excuse." Same way many people doubt when a woman cries rape. How many more REAL disorders, violent acts, etc., will be minimized as "excuses" simply because of the few whose situation may actually fit that term?
During pregnancy, hormones are extremely unbalanced along with the other changes to the human body. It is science, no assumption. Breasts are larger, blood flow is increased causing sensitive gums and bloody noses, exhaustion happens due to the rapid change in the body, and chemical imbalances occur. After delivery of the child or termination of the pregnancy, the body goes through yet another rapid change.
I will not deny, however, that there are women who may use their pregnancy as an excuse for abusive behavior. BUT, most of these women show signs of this behavior way before getting pregnant. Therapy focused on other issues may be in order. I've heard of stories of women going completely asinine and breaking through windows to get to a husband who has locked himself inside the house just because of her abusive behavior. That is a clear case of a woman who was in need of treatment prior to pregnancy. No doubt postpartum will occur in this woman.