Typically, they just don't care. It's not a glamorous or exciting case, and people who complain about dogs tend to nag and nag and nag, making call after call, so the local authorities tend to want to just make the whole thing go away. Very few people have the funds it would take to hire a defense attorney for their dog in a case like this, and truthfully, there aren't many attorneys who would take a case like this. Someone really should, though. It raises all sorts of ground breaking legal questions, the foremost in my mind being does our judicial principle of "innocent until proven guilty" (theoretically) hold true even when the defendant is of another species? It seems to me that if it is a sound and admirable principle it should extend to any entity within the bounds of our judiciary.
Charley once had a similar potentially ugly problem here at the farm before I met him. Someone built a new house one field over and had a smaller dog - about 30-40 pounds. Charley's old dog, Odd Job, was aging and had lost a great deal of movement in his back legs. The neighbor dog kept coming over and harrassing Odd Job, until he got tired of it and bit the dog pretty seriously. The other owner called animal control and told them that a pack of Rottweilers had mauled his dog. Animal control came out to investigate and Charley showed them the "pack of Rottweilers." Elderly Odd Job came limping out, walked up to the officer, licked his hand and promptly lost the balance in his hindquarters and flopped down. So much for a murderous pack of Rotties! Of course, the neighbor still holds a grudge. Charley has enough of a reputation as a loose cannon from his boxing days that no one really wants to press anything - especially regarding his dogs!