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.
I'd interpret the "minus first N per year" to mean that you get a few "free" fires per year - but after that your habits filling your closets with oily rags, overloading electrical extension cords, and smoking in bed will start to cost you.
Depending on who is paying and how much, someone might light a small fire to avoid the fine.