Leash laws being what they are, how can the county fine me for having a dog "running at large" when he was standing three feet away from me doing nothing? No, he didn't have a leash on -- but is that really running at large?
The county can fine you because leash means leash. Either Spot had one attached to his neck or Spot was a scofflaw.
There is an off chance you could persuade a judge that Spot was not, technically speaking, "running at large." Some leash laws require only that dogs be leashed or under voice command. Check your law -- and be prepared to prove that your dog freezes instantly whenever you whistle.