If the flight is work time, then the airport/airfield and the aircraft are all workplaces,
and the employer's WHS duties apply there, just as they would to any other workplace.
Consider also that something like "passengers must be, and stay, sober"
might be a T&C of the charter.
Seems reasonable on some of the smaller planes. An unruly and violent drunk passenger could be a serious safety issue for the pilot. It's not like a bus where you can stop the bus and call the police and have the passenger taken to goal.
A simple safety precaution and nothing personal directed at you. Suspect this is in response to previous incidents where miners have been drunk and caused problems on planes.
Actually no, the other crew miss behaved on their shift change and we got randomly breath tested because somehow we were at fault. (on site at the time)