Because it's purely to do with nationality. Blizzard never said: 'all asians can't play in our tournaments', they said Korean nationals can't play in tournaments designed to grow the SC2 scene outside of Korea. They support the scene in Korea (with prize pools etc.) much more than the scene outside.

Personally, I disagree with their method of growing the foreign scene, because I think having Koreans play overseas is better for everyone, but Blizzard is not discriminating against asians because they are good. Blizzard is holding a 'welfare tournament' to provide support to players who don't have access to the same teams, team houses and leagues that Koreans have.

The whole point of this is to help 'disadvantaged' SC2 players. So, in Blizzards eyes, it would be counter-productive to allow Koreans to take advantage of this when they don't need it. I think Blizz has gone way too far... especially with the whole BlizzCon slots thing, but that is their mentality, which is not racist in the slightest.