Whereas in Amsterdam, people want to leave their heads free because its mostly pretty cold, and you want to wear a proper head covering - like a beany or scarf - and this isn't always compatible with bike helments.
However, I've observed many times over people wearing helmets in Amsterdam. Its not a generalized circumstance that Amsterdam bike riders don't wear helmets - its just in this selection of pictures. Also, the average distances and speeds of the bike riders in Amsterdam are likely to be a lot smaller than those of Australia, where you have all the space in the world. A lot of people in Amsterdam don't ride much more than a kilometer or so, and at much reduced speeds than you would find on your average Australian road.