From the Netflix blog:
Some members use proxies or “unblockers” to access titles available outside their territory. To address this, we employ the same or similar measures other firms do. This technology continues to evolve and we are evolving with it. That means in coming weeks, those using proxies and unblockers will only be able to access the service in the country where they currently are. We are confident this change won’t impact members not using proxies.
It all comes down to the limitations of geographically-based licensing agreements.