I note the additional question: If amniocentesis really means that, why did Palin disclose that she had it? The answer would need to be that she wanted to tell the world that she knew she was about to give birth to a child with Down Syndrome and wanted pro-Life credit for her... I was going to say decision. Which brings us back to Sullivan's question.
(Sullivan has more here, where readers have attempted answers.)
ADDED: This question is being discussed on Metafilter, with many good comments, especially this from anastasiav:
And if, for whatever reason, the parents are sure that they will not abort the child, then there is no point in getting tested because the information gained is of no use.
I'd like to speak to this. I was tested. If our son had Down Syndrome I would not have terminated the pregnancy (although I might have if some of the more horrific abnormalities that are possible had been present) -- but it was important to me to know. In advance. Not find out the day my perfect and dreamed of child was born, but far enough in advance to plan and learn and grieve if I needed to. Far enough in advance so that everyone would know and no one would recoil in surprise and then try to put on their "oh what a lovely baby" mask. Far enough in advance to be able to understand what I was getting into before I was standing in the middle of the field.
Its commonplace now to learn the gender of your child in advance "so you can plan." I see this testing as no different. Its a way to be prepared. To plan.