Arson is a felony. Death incidental to any felony, whether recklessly or maliciously committed, is viewed as felony murder in some jurisdictions.
It doesn't matter if you intend for someone to die, or even for instance in arson, if you knew anyone was in the building. Another example is armed robbery. If three thugs rob a store and one of them shoots and kills someone in the store, they can ALL be charged with first degree murder.
Accidental death or lack of intent are not defenses. So, I hate to say it, but I agree with the ten years Ted Maher got.
I did a quick online search, and here is one case in Rhode Island from 2000 that talks a lot about the doctrine. http://www.geocities.com/wrongfulconvictions/diaz/felonyMurder.htm