Halliday & Matthiessen (2014: 341):
Except in the special case of the medio–receptive voice, the Medium is obligatory in all processes; and it is the only element that is, other than the process itself. … The Medium is also the only element that is never introduced into the clause by means of a preposition (again with the same exception of medio–receptives); it is treated as something that always participates directly in the process.