I used to have a theory that you needed the genre to form before you could get parody of the genre. So, in the simplest form, you need Star Wars and space fantasy before you can get Spaceballs. Which is obviously true in that case--but doesn't really explain why Dashiell Hammett's hard-boiled noir seems so much more light-hearted than Raymond Chandler's knight-errant detective stories. Chandler's heroes are by turns tragic, Christ-like (in accepting sins to protect others), world-weary, and solemn. Hammett's heroes are paunchy, quick witted, and nimble with their principles.
In some way, that sums up the tone of "Arson Plus." For instance, when a sheriff's deputy accompanies the detective and kills a man, the deputy says,
“I ain’t an inquisitive sort of fellow, but I hope you don’t mind telling me why I shot this lad.”And the detective tells him what he figured out about this strange arson case, which looks like it'll be arson plus homicide, but turns out to be arson plus insurance fraud. But as for the killing or any of the human tragedy that led to this point--eh, that's someone else's department.
In fact, except for the fact that this takes place in the streets rather than in a cosy tea-room; and includes some fast-talking dames and a private eye and guns; this story reads in many ways like an Agatha Christie problem story: a house burned down--whodunnit?
And maybe that's one of the keys to Hammett's lighter tone. Whereas Chandler is more interested in the morality of why, Hammett is still interested in the mechanics of how the crime was done. And that's my explanation for why this story reads so character-light and so plot-heavy, with the detective gathering all the evidence being the bulk to the text.