No disrespect here and the usual disclaimer of me being Pax - they ferried it empty home to FRA - so, if it is considered airworthy why not loaded then?
Any numbe of reasons it could be airworthy for a ferry flight but not
revenue, just a couple that come to mind:
A:the damage "could" cause a pressuration issue which would be a non-event with just a crew but not an ideal situation with a full load of payring customers.
B: The door was damaged and needed for emergency evacuation.
C: Engine not useable so ferry flight on 3 of 4.