I think the point they are trying to make is that there will still be fires started by the arsonist, so celebrating a single fire being put out while the arsonist is likely busy trying to start another fire looks bad. Even though, as you point out, the Fire Department isn't going to be making any arrests for arson, that's the Police Department's job.

I'm not sure the analogy applies perfectly anyway, since it was "only" a single "fire" in which "only" 700,000 victims were affected.