It is not unusual that a PSO operation sees the aircraft sitting on the ground for a long time (see, for example, DND-STN). As the operation is subsidised, normal rules of economy do not really apply. Plus two rotations daily probably makes crewing easier and there is no risk that additional flights at the airline's own commercial risk mess up the PSO operation (my guess is that there are penalty clauses in the PSO contract).
Last edited by virginblue; 24th Feb 2017 at 14:18.