Aw, cmon - no one owns the picture space and no one was victimized. And just because someone's having a 'special moment' doesn't mean someone else can't have some fun - maybe that was the bomber's special moment. Now, if the guy starts to follow them and goes out of his way to ruin pictures (jumping in front and such) then yeah, he's a jerk. Otherwise, I disagree.
Assuming he just turned and hammed for the camera, then he's just as entitled to be there and do as he pleases, as the people posing and the camera holder do. In fact if anything, it is the people taking the picture that are the ones imposing on others as they demand clear space between the camera and those posing. We generally don't worry too much about that, but if everyone did it all the time, it would really impact most people's enjoyment of the parks.
If someone points a camera at me and shoots, well, I get to pose as I please. If they don't like it, they can wait until I move on, or move on themselves.