One day when I played this scene I wondered it too.

I think he has a major crush for Maria. Maybe he was doing it for Empire. Maybe Maria was too anti-Empire or Gestahl wanted Maria to sing for his soldiers. Werent there some mention that Setzer worked for Empire? Not directly but at least he wasn´t against Empire in public.

And also where did real Maria disappear? Or was she in reality Siren and thats why she was unavailable for Opera.