Halliday & Matthiessen (2014: 225):
… Actor and Subject are distinct in a ‘passive’ — or ‘receptive’ — clause … Here the Actor is not interpersonally ‘charged’ with the rôle of Subject, but is rather given the lower status of Adjunct and can thus be left out … We therefore have to be careful to distinguish the experiential notion of ‘the one doing the deed’ (or ‘the one bringing about the change’) from the interpersonal notion of ‘the one held modally responsible’ (or ‘the one given the status of the nub of the argument’).