Bureaucracy. Generally doctors are the ones that need to give PBS sanctioned (or whatever it is) scripts so you don't pay through your nose, and instead get everybody else who pays taxes to do so for you.
Also, the government feels a need to know what medications you're taking. Responsible distribution of resources and all that jazz *cough*
These days, however, since alternative medicine is on the rise, I expect discretionary spending on naturopathic etc. etc. goods and services to have gone through the roof.
What bothers me is that every medical administrative system interface is spammed with specifically designated pharmaceutical advertisements even worse than almost every non-premium site on teh internetz. Next time you're in a doctor's office, try take a squiz at their computer screen. It's horrid.