I don't know why Japanese do this, but it's almost a law that they jump up, kick their heels and flash double peace signs whenever they get their photos taken at some tourist spot, even if the "tourist spot" is a battlefield where hundreds of Japanese soldiers perished in a banzai charge or is the Arizona Memorial at Pearl Harbor. I've even seen them do it at the suicide cliffs memorial on Saipan, where you'd think they'd realize acting like a jackass is inappropriate. But nope.
Oh, well. Better them than us, right?
