Surely anyone judging the parents actions is just being judgemental. None of us were in their shoes so we are in no position to give our arm chair judgments. We do not have all of the information that they had at the time they acted the way they did.
I don't know whether I could have done what they did. I hope that if I felt so strongly about something I would act in accordance with my head and my heart despite the judgements of other people.
Personally I do believe they thought they were acting in their child's interests. And I admire them for it. This is not a religious issue and our negative opinion of the Jehovah's Witness religion shouldn't have ANY bearing on this case. As the OP stated, they were probably acting independently of any JW advice as well as medical.
I'm with SlimBoyFat on this. I distinctly remember quality of life being the main issue - not survival in itself.