I mean you can’t put a building on fire and say you should not be sentenced for the whole building burning down because the impact of the response by the fire department (if they had been faster/better the fire would not caused so bad damages)
The building owner has a degree of duty to mitigate loss. They can't go around opening doors and windows after the fire is started but before the fire department gets there and be all "whoops not my fault blame it on the guy who started the fire" regardless of how the fire started.
Actually if only half the building burns down you aren't supposed to be sentenced as if the whole building burned down. Even if the whole building would have burned down if the fire department didn't show up.