When Japanese film actors speak fluent English

I find it odd (and annoying too) for Japanese film actors to speak fluent and coherent English when playing Japanese roles in Western movies. For example, in The Last Samurai or Silk, those Japanese roles speak English fluidly despite the fact that a coherent English is something you would rarely heard from a Japanese’ mouth, even by today’s standards.

Suppose that it’s for the world audience’s understanding. Then why did not the Western actors speak Japanese instead, and putting subtitle in order to be understood?

By speaking English, those historical Japanese (albeit Hollywood-made) movies had lost their real nuance. They gave in what was supposed to be originality for the sake of audience comprehension.