Why? The goal is to incentivize property owners to fix broken systems not to pay for the actual cost of responding to a real call. That's covered by taxes.
You never want to supply any reason for someone to demur about reporting a fire.
Not true for all places. There are places where the callee can be charged for the costs of a call and the owner of the building has to pay the firefighters (well, his mandatory government-issued insurance will pay). It's moving bills from one stack to another, but it's not free as in beer.