As for CAR 226 (1) (c) sorry CASA. On MY flight deck if it is convenient for the F/A to sit in a control seat while the F/O, or I, go back for a leak, then I will authorise it as Captain. I will invoke (2) of the same CAR as justification. The F/A is far safer strapped into a seat should the unexpected happen during the absence of one of the pilots
Because most jumpseats on narrow body aircraft merely get in the way it is easier to drop into a control seat and strap in. Think depressurisation, turbulence etc.
A rule that can not be enforced invites contempt. That saying “rules are for the guidance of wise men and the blind obedience of fools” comes to mind.
Change the wording from ‘CASA authorises ‘ to ‘Pilot in Command authorises’ and the whole silly argument goes away