The exemptions always expose the safety justification for a rule as a sham.
We won't fit the RAAF fleet because it's too costly, and that's safe.
Heavy metal can fly around for days without it serviceable, and that's safe.
But everyone else has to have it fitted and serviceable, irrespective of the cost, because to do otherwise would be unsafe.
It would be silly to expect the taxpayers to pay for the fitment of ASD-B to the legacy RAAF fleet, but safety demands that those same taxpayers pay for the fitment of ADS-B to a legacy GA aircraft if they happen to own one.
As with almost all other aspects of Australian aviation regulation, the ADS-B rules and exemptions are just another manifestation of political expedience hiding behind the facade of safety rhetoric.