Why is StarCraft 2 divided by regions?
I sent an e-mail to blizzard support, this is what they answered:
Quality and customer service are the highest priorities for Blizzard Entertainment. Because we cannot guarantee a positive gaming experience for users connecting to servers outside the supported region, we do not support use outside of the designated region. You can change which region you are purchasing for by clicking the (change) link next to the region. I recommend that, if possible, you acquire a localized version of the game for a more positive gaming experience.
In other words, players in one region of the world will not be able to play on alternate regions at this time. This is something that we will continue to investigate as a possibility for the future, however, if you have purchased a version of StarCraft II from outside of your home region we recommend that you return it and purchase the localized version instead.
Please also keep in mind that you will not be able to access your account if you are using a different regional game version (other than your home regions version). For example, you will be unable to connect to the North American realms if using a Chinese, Korean, European, or Taiwanese version of the game.
We recommend that you continue to visit http://us.battle.net/sc2/en/ for the latest announcements and updates on this and other StarCraft II features.
If you have additional questions or concerns, please reply to this email. If you would rather speak with a representative directly, please contact Account & Technical Services. Our contact information can be found at http://blizzard.com/support/article/cs.
The short answer is because Starcraft 2 is region-locked, like most videos, games, and game consoles.
The easiest way to circumvent this region locking is to simply purchase another copy of the game from the region you're seeking to join.
The patient route is to wait until Blizzard gets around to implementing cross-region battle.net.
Edit: Too bad you don't live in Australia: http://www.kotaku.com.au/2010/07/starcraft-ii-will-not-be-region-locked-in-australia/
The another reason is maybe because of different pricing policy made by Blizzard. For example unlimited euro version costs 60 euro, while russian one is twice chipper. Maybe somebody will be unpleased that he had to pay more then another for the same server.