Then why wouldn't you do it to a child with 'normal' intelligence?
-A8
Because (1) in the case of no particular physical disablities, it is not necessary to do it in order to enhance the life of a child with normal intelligence. (2) In the case of a child with normal intelligence but even severe physical limitations, that child has capacities that the child in this article will never have: the capacity to give consent or withhold it, the capacity to think for him/herself, to form mature relationships.
In a child with normal intelligence, making this choice on their behalf would deny their capacity to make choices for themselves.
This child will never have the capacity to make choices or exercise free will. Therefore, the decision should rightfully be made by the parents as to the best path to take to make that child's life as comfortable and love-filled as possible. So they did.
A child with normal intelligence is not going to rely upon me for care. As children grow bigger, they naturally take on their own physical care, etc. It simply isn't necessary or practical.