Spectators got to participate in the run, whether they planned on it or not. After watching their runner finish, the route for spectators to get from the finish line to the family meeting area was even longer than that for runners and required going through multiple security checkpoints. Seeing spectators running down the car-free streets was not uncommon as the race progressed.
Some people like having their photo taken. This man was one of them. Once you've stood in the rain for hours on end, you can choose to be miserable or enthusiastic. He chose the latter.