Funny how that works.
If the City owned an aircraft, and kept it in a hangar that was owned by the city on the airport that was owned by the city, it would still have to pay rent to the airport that is owned by the city.
Why does it work this way? Because the federal government finances airports, and that make them public property. Not even the city can use the property on an airport without paying rent for the land lease.
From the Federal Register .
B. Prohibited Uses of Airport Revenue
Prohibited uses of airport revenue include but are not limited to:
9. Use of land by the sponsor for aeronautical purposes rent-free or for nominal rental rates, except to the extent permitted by Section VII.E of this policy.