I doubt many non-lawyers know precisely what the penalty for arson is. However I do think it's general knowledge that arson is considered a serious crime with serious penalties. Arson in the general case is easily the worst sort of property crime since it often goes hand-in-hand with murder. Granted, murder wasn't really a plausible outcome in this particular cause, but the general association between arson and murder is nevertheless commonly understood and arson is consequently understood to be a very serious matter, even when nobody gets hurt.