Yeah, that's a bit of a problem. I think you'll recall my post a while back about how all the handbooks of arson investigation, particularly regarding various accelerants, are being rewritten because many were based on anecdote, not science. The most damning was the discovery that what investigators had previously classified as accelerant trails on the floor were actually a peculiar distillation effect of the hot gas from melted plastic furnishing condensing when they met the cooler floor surface. They discovered this after one of the many test fires they set at the nearby Fire Institute at UMd.
People were executed on the basis of faulty data that "arose" from examining test fires from a time when there was very little plastic in a burning home. So the arson investigation profession took a big hit, along with a number of other forensic profession as more and more judges are requiring scientific and not tradition or simulation based testimony. You may have heard of Texas' recently discredited "dog lineup" that has been laughed out of court - finally.
Somehow, a lot of these guys fail to detect the causes of deliberately set fires which makes their "hit" rate in past cases very suspicious. Reminds me of why cops score very high on detecting deception: they believe everyone's lying. Their apparently accuracy as human lie detectors is offset by their inability to detect the truth as often as other people.
Many accidental fires look very much like arson and vice-versa. I've sat through the testimony of a lot of fire and accident reconstruction specialists and have only been mildly impressed by some and complete flabbergasted by the chutzpah of others when it came to try to assess fault. Perhaps the prosecutors find more credible witnesses for criminal cases but I doubt it. As OJ showed, with enough money a good defense attorney could discredit Diogenes and Abe Lincoln both.
These cases are hard to prove (beyond appeal) because it's rare they can find a witness or a CCTV recording showing the defendant setting the fire. After a decade of CSI watching, jurors come to expect evidence that exists only on TV shows. No witnesses, to a good lawyer, is almost all the reasonable doubt you need. As you noted, when the crime scene is blown all to hell and evidence has not been collected with the same care as it would in a typical murder crime scene, you've got another evidentiary hill to climb.
These people were out of town, apparently, when it happened. Making it even harder for a jury to convict unless a timer device with a direct connection to the defendant is found. Plenty of people I've seen post on Usenet wouldn't hesitate to "Ben Quick" one of their neighbor once they realized they weren't home. As other posters have noted, some of those evil people have immolated themselves along with their target. As my WWII vet editor told me: "Never underestimate stupidity." If in doubt, see the thread where two of our regulars are arguing who is more stupid. (-:
-- Bobby G.