(a) Australian culture being quite egalitarian, ie people there see other people just as people rather how much money or power they have, etc and
(b) American management being more of the power tripping variety, so they're more likely to wield their power in a threatening and retaliatory fashion, and so underlings tend to be more deferential, including the at-will employment, as you say.
Power tripping isn't really about being "tempted by power", rather it's when you become self-righteous because you got granted (some) power.
If you want some sweeping generalisation for why Americans are more prone to this sin, how about the various old cultural patterns of thought that fit under the header "manifest destiny"?
Australia is a colonial nation that unified its entire continent - that's beating the US, who have two large neighbors and a couple small ones.
(Of course both countries have indigenous nations inside them. They're actually very similar, which I think is the reason Australians tend to be culturally resentful of the US on surveys in a way almost no other country is.)
(a) Australian culture being quite egalitarian, ie people there see other people just as people rather how much money or power they have, etc and
(b) American management being more of the power tripping variety, so they're more likely to wield their power in a threatening and retaliatory fashion, and so underlings tend to be more deferential, including the at-will employment, as you say.