AussieAviatior, the reason for the anomaly is in a very simple part of the regulations which currently prescribe that only aircraft above 30,000kg MTOW carrying out RPT operations require to be screened.
This is being expanded to all aircraft of 20,000 MTOW and above from 2012. Charter and other non- RPT operations are being looked at to be included in this but nothing has been decided yet.
As for the PMs aircraft, military aircraft are classifed as "state aircraft" and exempt from screening requirements.