I'm surprised that all these WATB canophile lawyers seem not to be familiar with Seattle's Animal Control Code:
Unless Dan and his al fresco dining companions happened to be sitting in a part of the park that was a designated off-leash area, it was that dog's owner who was in the wrong.
And seriously, if you don't understand that leash laws protect our pets as much they protect the public, you have no business owning a dog. If an unattended and unfamiliar dog approaches you, and you reasonably believe it poses an imminent threat to you or those around you, you have every right to kill it on the spot.
So if that dog that Dan shoved away had bared its teeth or snarled or lunged at anyone or done anything else even remotely threatening, instead of a shove, Dan could have kicked it in the head, or beat it with a baseball bat, or stabbed it, or slit its throat, or pulled out a licensed firearm and shot it right between its pretty soft doe eyes. And still he wouldn't have been in the wrong.