To quote Jim :
"
There are many that think the investigation should be bifurcated. This means separated into two branches or parts.
The first would be to determine what was the source of energy that allowed ignition to take place and what was the competent fuel that was first ignited.
The second part is the motive, intent and classification.
Here is where there is a difference between the task required based on the investigation being conducted for the public or private sector.
It is more likely that the public sector investigator will be asked to develop the motive and intent than the private investigator.
Most insurance companies do not request the private investigator to conduct sufficient background investigation to determine motive and intent.
"

From where I stand, i.e. an ex-forensic scientist now FI standing outside the US looking in,
it appears as if your public-sector FI's are playing FI & detective
(over here, public FI's fall under the state forensic services, straight bifurcated FI only)
which, in my opinion, is a clash of interests;
many of Fire Investigation's shameful cases coming to light are due to an over-zealous public FI trying to nail his man/make a name for himself.
A hint of misplaced emphasis is perhaps in the name of the IAAI, i.e. investigating "Arson" instead of "Fire".

Not that I am particularly against the inclusion of this Chapter, red flags & all;
921 is a guide to the Insurance industry and detectives as well, it's readers can decide how much advice to take.
Personally, as a straight-up forensic FI, I pass any such "red flags" on to my client to investigate;
my task is fire origin- & -cause, not -classification, motive, means or opportunity.