I think NFIRS is even more vague than 921. Here is the only language or guidance given in the NFIRS 5.0 complete reference guide on cause of ignition:
"CAUSE OF IGNITION CODES
1 Intentional. Includes deliberate misuse of heat source or a fire of an incendiary nature.
2 Unintentional. Includes fires caused by careless, reckless, or accidental acts.
3 Failure of equipment or heat source. Includes mechanical problems.
4 Act of nature. Includes causes related to weather, earthquakes, floods, and animals.
5 Cause under investigation.
U Cause undetermined after investigation."

So the question becomes...What exactly is "deliberate misuse of a heat source"? If someone brings their grill inside their garage because it's raining and that burns down the garage, is that an intentional fire? What if the product manual specifically says not to use indoors? That person made deliberate act to bring the grill inside and in doing so, misused a heat source (the product).

-Scott