I was initially surprised and pleased to hear that several left-wing groups (Labor Party, B'Tzelem) had called for a inquiry into police brutality at Amona. Perhaps they actually had some principles and were willing to fight for the rights of the extreme right as well as the extreme left.
But then I realized that no, this was just another transparent political maneuver. Anything which makes Kadima and Ehud Olmert look bad will deliver votes to the smaller parties - Likud and the religious parties on the right, but also Labor and Meretz on the left. It can't look good to order the police to go out and unnecessarily smash skulls.
Or, to look at it more cynically: The Amona protestors effectively declared themselves to be at war with the state of Israel, and B'Tzelem at least has always considered its job to protect Israel's enemies.