When going through this procedure, the woman giving birth is going to be going though a huge amount of pain, and will need to take the sedative before delivery so that she does not have to suffer as much the entire time. The mother needs as much help as possible.
Those who advocate for natural child birth might say that no sedating medicine should ever be given to a mother at the birth, but we have not seen any problems with those of us whose mothers received it. The health and well being of the mother is an important factor in a child's early days.
When a Caesarian section happens, the mother doesn't need to be awake to deliver the baby. As such, a sedative before delivery should be completely appropriate so she relaxes, so long as the child isn't affected while still in the womb. Anesthesia can't be applied too much because it may harm the baby while still in utero.
A caesarean section is a surgery. Mothers want to be awake for the birth of the baby whether it is by C-section or vaginally. The doctors cannot risk the chance that the Mother may not be calm or may be combative. The sedative allows the Mother to be awake and calm during the birth of her baby.
I personally have had to caesarean sections in my lifetime. It was by no means a pleasant experience and not something that I would like to have happen again, but I don't think that there needs to be a sedative added. I couldn't imagine not being awake and aware when my child was born.