Tom Bartlett On Initiator Vs Actor

Tom Bartlett wrote on Sysfling 

In IFG2 (pp.164 and 171) Halliday analyses "the nail tore the cloth" as "the nail (Ac/Ag) tore the cloth (Go/Med)" but "the police exploded the bomb" as "the police (Initiator/Ag) exploded the bomb (Ac/Med)". I cannot follow his argument as to the Initiator/Actor distinction between the nail and the police here. Both clauses can be paraphrased with MAKE: "the nail made the cloth tear" and "the police made the bomb explode" and in both cases the omission of the Complement changes the Medium: "the nail tore" and "the police exploded". Can anyone explain to me the linguistic reasoning behind the different transitive (cf. ergative) analyses?

Blogger Comment:

Because the nail tore the cloth is a transitive material clause, it is Actor^Process^Goal.  It is transitive because the tearing process performed by the nail extends to another participant.

Because the police exploded the bomb is an intransitive material clause (the bomb exploded) with the feature of 'cause' added, it is Initiator^Process^Actor.  It is intransitive because the exploding process performed by the bomb does not extend to another participant.

Halliday & Matthiessen (2004: 292):

In a material process, [the Agent] is the Actor — provided the process is one that has a Goal [ie in a transitive clause]; otherwise [ie in an intransitive clause] it may be present as the Initiator of the process.

See also here.