Because Primary Schools will often ask for an immunisation certificate. Unless you have already chosen not to immunise, most people assume this means it is compulsory - but we in the know realise you can simply lodge a conscientious objection with the school. Education is another of those government things that they are pretty much obliged to provide, whether your children are immunised or not.

Oddly enough, the only place where I have found that non-immunisation is a problem, is for adults working in health care. NSW Health have just introduced a new policy that requires all staff to have immunisations to work in certain areas, or they can be permanently reassigned to other areas, even areas outside of their training. That's coming on pretty heavily! As an adult, I have no qualms having immunisation boosters, etc., but my wife (who is a midwife) is allergic to some of the components of the MMR. She had a life-threatening reaction as a child. When she told them that that was the reason she wasn't getting immunised against measles/mumps/rubella, they actually suggested that she come into the ED and be immunised there - so that they could resuscitate her when they were done!