[Traincustomer]

I'm not sure if the Dublin Bus Nitelink is classified as commercial or as part of the public service obligation but either way buses would often be running along part if not most/all of a route on the way back to the garage/for the next run (where applicable) so better to earn a bit of revenue whereas an empty out of service bus earns a guaranteed zero.

Incidentally the three Bus Éireann Nightrider routes from Dublin operate in both directions.