I don't think this is unusual. I've flown many intra Schengen sectors without passport/ID card check at the gate. On a flight from STK to AGP last year with SAS I used my Lufthansa Miles & More membership card in a self-service check-in machine as the only identification. The machine printed by baggage tag which I then attached and sent it through an automatic check-in desk. At security used the same card in a machine which printed a document the size of a bus ticket to confirm my seat allocation on board and was the only thing I required to board. No photo ID, and a fully automated procedure.