Because the left embodies the only hope for combating this whole disaster, and the left is very small and very weak and totally dependent on being ethically superior to the right. Hence, when a dolt like Churchill uses his relative fame to endorse and re-endorse Al Qaeda and the targeting of civilians, it hurts the only chance human beings have for solving this problem. The supposedly Eichmannic population of the United States already understands that it's wrong and illegal to target civilians. If the left starts talking like Churchill, they will lose that understanding and side with the right. Part of the left's job is to insist on preserving and expanding the principle, while searching for ways to crack open the media system that keeps people from connecting the dots about "our" own actions. It's a very tough job, but it's the only one in town.
Why can't you see this? You don't seem to be one of the loons who think Churchill's statement helps us.
I'm all in favor of staying on guard against racist attacks, but this isn't a racist attack. It just isn't.