I'd assume that because everyone here is from a different but nearly identical world to those that their friends are from. So when you're here, you're considered "the" criminal Kamui from your world, unless there is a Kamui whose actions differed significantly from yours, and thus they had to be put on trial a second time. It would be pointless to arrest an infinite number of Kamuis from an infinite number of worlds, most of them exactly the same.