My hope is that this isn't intended to be a profit generator; that the city has actually analyzed the statistics and said "we can collect just enough of these $75 fees in a year to cover the actual costs of extinguishing fires in the county for the same year". Based on the way they answer questions, I don't think this is the case.
The purpose of running an insurance company is to generate profit. The purpose of running a fire service is to protect lives and property. I'll let pundits argue whether it is ethical to generate profit from selling insurance, but it is certainly not ethical to purposely run a taxpayer funded public service agency for the generation of profit (revenues exceeding expenses). I would have less of a problem if the FD simply said "we don't respond outside of our jurisdiction".
Many ambulance services have a similar fee to this FD, but with one major difference. If you didn't pay their subscription fee, they will still respond, treat, and transport, but they'll bill you for the full cost. If this FD wants to shift to an ethically supportable policy, this is what they would do. They won't, though, because that would decrease the number of $75 fees they can profit from.