I think it's fair to take this approach. If a person is mentally impaired and unable to care for a child then I think it's warranted. The parents made the right choice on this occassion I would think.
My dads youngest sister is mentally retarded and she had a baby whom was removed from her care when she was 5yrs old. My dads sister has the mental capacity of an 8yr old, she could not provide the care that child needed. It was the right thing to have her removed from her care and placed with a family that could care for her. Her parents having had her sterilised to begin with would have been the right thing to do.