Car seat question Aus -Uk USA?

I have just been given a copy of the new car seat laws from my maternal and child health nurse which became effective in nov 09. I have seen heaps of questions on here about leaving their child rear facing and it being the law in Uk and USA sometimes up to 2 yrs old. I am in Australia and in this book it states that Babies from birth- 6 months must be in a rearward facing carseat. From 6 months until four yrs they Either be in a rearward facing OR forward facing seat depending on weight.My rearward facing seat is only till 9 kg 4-7 years must be in a booster with a lap sash seatbelt or harness. It also says that if the child reaches the weight limit for rearward facing seats it must be turned around. My question is Why such a big difference in laws here to wherever you are?? I have answered questions before about this and have been given heaps of thumbs down even though this is our law here!