Who was responsible for letting them on in that state is the question?
Ground staff are inevitably in a hurry to get the doors closed and are usually ready to look the other way and palm off an inebriated passenger as simply having had "one or two."
With the door open, the responsibility for organising the punters rests with the ground staff (at my outfit anyway).
Maybe if the gate staff had licences to risk, they'd be more responsible with regard to whom they allow onto the aircraft.