Halliday & Matthiessen (2014: 349):
If the clause is effective, since either participant can then become Subject there is a choice between operative and receptive. The reasons for choosing receptive are as follows:(1) to get the Medium as Subject, and therefore as unmarked Theme … and
(2) to make the Agent either(i) late news, by putting it last … or
(ii) implicit, by leaving it out.