er... thought you were kidding at first, so deleted my first post.. but...
I guarantee you at least 99% of viewers would have no idea nor care that Cho is korean rather than Japanese.
Furthermore, unless its documented explicitly somewhere, no one knows what the hell Sulu's technical ethnicity actually is.

(I suspect this is the strongest point of my arguement. I mean, how much can you explain and/or defend the ethnicity of a fictional TV character).
just sayin'. Finding a specifically Japanese actor who could carry the role may well have been extremely difficult if not impossible.
Anyway, if you were just joking around, do forgive me for blathering on so.

I just couldn't let it rest, as it seems to me no real cinematic crime has been committed here :p