Once again, This Is Not About Intellectual Property. This is about the fact that the prize is a computer, which could impose an import duty on the recipient, and/or cause Turbine to incur liability which they cannot necessarily legally limit in other nations. If the prize were Turbine Points (or something good, like a mod which lets the winner access Devotion enhancement as a Ranger or Pally under the new enhancement system), there would be no issue.
Interesting. So they intentionally decided not to make the contest available to all their customers when they went for a computer as a prize.
Or even provide a secondary choice for non-US citizens.