Yes, there is the Japanese Triathlon Union, but I would not be able to read any rulebook they had even if there was one.
I just don't get the point of the shirt. Women don't wear shirts, they just have whatever their swimsuit covers. It must be some kind of modesty issue; I can't imagine what protection a skimpy top would give in a crash, especially when many people still wear speedos (over here anyway). I'm surprised that you've never seen anyone without a shirt... maybe I'm the only freak that would like to do this.