Sort of an odd statement considering Jackie Chan just won an honorary Oscar today.
Think of it this way: Ghost in the Shell is an anime adaption. Do you see the anime studios in Japan going out of their way to hire american voice actors whenever a character is american? If they don't, then why should US movie studios?
This notion that for whatever reason an actor can't play someone of a different ethnicity is a very recent sensitivity. Besides her obvious acting talent, they most likely picked Scarlet Johanson in the hopes that she'll bring a larger audience considering how popular she is among their intended audience.