When the movie came out, there was a controversy from certain quarters because most of the Asian roles are played by non-Japanese. In my anecdotal experience, the Japanese-American and Japanese expat community did not think this was a problem, just because East Asians look similar to each other, so why not cast non-Japanese in some roles?
If you're Caucasian, turn the tables. You might ask the same question of a movie with an all-European cast: do you think Braveheart cast only Scottish actors? Of course not! Why? Because West Europeans look similar to each other. Dienekes gives the results of a quiz to see if people could guess (if you want to test yourself, don't look at the top of the article because the answer is there; now here's the link.) Did you do any better?
It would be really interesting to have a bunch of pictures of Europeans - and Asians, and people from everywhere else for that matter - that the crowd can vote on to see if we really can tell a difference between adjacent nationalities. My guess is usually not.