It's not just with firearms. It's anything that sells papers or clicks or whatever. And it's especially true of local news outlets because after you run out of "fireman rescues treed kittens" stories you need something spicier.
After receiving calls - unsubstantiated nevertheless, from frightened people making claims of physical danger, the police HAVE to investigate. Once they have cause to do so, they should take care not to damage what they touch. Laying out firearms on grass is abuse of private property in my opinion. And especially piling them up in the back of a truck like so much cord wood.
As to the inert explosives, they didn't know until examined by experts - and if there's no immediate proof available that they are in fact inert, they MUST assume there's real danger. Think what the chief of police would have to say if one of those bombs was live and they didn't take any action. This I suspect is what really drove their actions.
Having said all this, and seeing the final outcome is nothing dangerous or illegal was found, the people who reported to police should be sued in civil court for damages, imo.
The police department or town may or may not have liability, I'm not a lawyer, but they had imo no grounds to remove anything that was clearly legal in that state. For this they fail the ethics test.