Then there are the cases where the building codes are not observed; Wikipedia even has a category "Fire disasters involving barricaded escape routes" (http://en.wikipedia.org/wiki/Category:Fire_disasters_involvi...). Only one of these is listed as an arson attack, I know of one other (Happy Land), although both of these were code type problems. Hmmm, and according to Wikipedia, the Dupont Plaza Hotel arson, 2nd worst in US history at a minimum of 97 killed, locked emergency exits on a casino floor were a factor (wow, I'm impressed Wikipedia's article does not use the word union once...).
Still can't find a list of arson fires, so further analysis by me will have to wait, but I know plenty of smaller scale ones I've read about have involved the arsonists blocking the entrances, often by setting more than one fire.