"Who can defend such a thing?"

Physicians, who promise to do no harm. Apparently, the physicians in this case have decided further treatment would cause harm. They did not decide it would be too expensive. They did not decide the child was superfluous. They decided further treatment would hurt the child.

The parents have a different opinion, essentially that anything that keeps the child alive is less harmful than treatment that leads to the child's death. That certainly sounds reasonable, but surely you can allow that a physician may decide otherwise without becoming unreasonable.