Actually reloading is a hazard ...
... which is why there is a hazmat fee associated with gun powder and primers. I think the cop was correct, reloading in a confined space is unsafe, but then so is a propane heater, stove, natural gas heater, range/oven, electric space heater, iron, electric lighting, ...
I can understand the landlord. I suspect she does not want the place to burn down and then find insurance will not cover it, and/or she has law suits for having such an operation taking place on her property.
Perhaps she thinks of reloading as working with an explosive. Gun powder is not an explosive. I think gunpowder certainly adds to a burning situation though, and in the 'right' combination of closed space, air, and powder what appears as an explosion takes place. I do not even think a 'Ka-boom' is really an explosion.
as much a pain as it is to move, I support the property owner's right to not go alone with reloading on the property. Too bad the cop opened his/her mouth. But I suspect the landlord was looking for a supporting answer such as that. I see no reason to 'counterstrike' as seems to me to be suggested by some posts. The landlord does not like gunpowder/primers stored on the property, so be it. Why battle her ? Life is too short to adopt an attitude of, "I'm right, you're wrong; I'm smart, you're stupid; I'm strong, you're weak ..." Enjoy the journey, even if that means you gotta move.
Good luck in your new place.