It's not really restricted to Trek, it's a phenom for all of Hollywood. I remember reading something a few years ago that said that the reason that Ken Watanabe is the Japanese guy in so many films is because there is a limited pool of talented actors (men) that are of Japanese descent in Hollywood.
But yeah, as others have said, a big part of "acting" is making believe you are the character or something.
With Trek they do make a conscious effort to appeal to diversity in Star Fleet. Disney's Mulan had primarily Asian actors of non-Chinese decent because they couldn't find enough actors to voice the characters.
All you have to do is look at the recent Cloud Atlas and it's accusations of 'yellow face' for casting white actors and using make up to make them look asian, and depicting stereotypes of asian culture. Honestly I think the Wachowski's are a duo who have seen too much anime, and think it's edgey and cool to depict their interpretation of it on the big screen. Look at their movies if you doubt me.
The same thing can be said about M. Night Shymalan's The Last Airbender and it's criticisms of racebending. He cast white actors for all the asian and inuit roles. If given a sequel he said he would make sure that that the last character to the Airbender "Toph" would be an asian actress. M. Night's defense for his casting white actors for these roles was that "anime is ambiguous" and thus the actors race didn't matter.