they are by your own definition fundamentalists. they just have a different conservative mantra. the left is I think more fundamentalist and conservative than the "conservatives"
they have an unchanging platform of views which is completely based on nonsensical facts. a religious devotion which leads to suicide for political aims
do you honestly think the Rachael Corrie's of the world are not religious extremists?
look at the idols of the the far left
Che Guevara, Mao, two mass murderers
the left has a supremely rigid thinking process
you can fill in the blanks.
just substitute the way of thinking and the rigidity of thinking instead of looking at the content
its why radical ISLAM , a completely dogmatic viewpoint of the world is often allied with the left in political movements.
and the thought pattern is the same, since Islam also finds great truth in David dukes. so its the way of thinking, not the content that defines fundamentalist