Because at the back of the minds of the Aussie kiddies doing the assault, they expect that they will get away with a light fine (say, $50) and verbal reprimand.
They would not dare to try it here in Singapore. Because we may cane them 24 strokes on top of a 5 year jail term.
Another way to solve the problem in Sydney would be to enact the "Stand you ground law" that Florida in USA has, and allow people in Sydney to carry guns with a licence.
Then these two victims could have fired their Magnum 44 pistols into the gang and say, "Go ahead - Make My Day"
(for those who are too young to know, this is a reference to the movie Dirty Harry)
[video=youtube;Flt9K8vlJGE]http://www.youtube.com/watch?v=Flt9K8vlJGE[/video]