When you target UIRoot, it just falls through to targeting the camera used to draw the UI. In your case the game view is hidden, and I'm venturing a guess that you're on a mac -- and there is a Unity bug preventing the screen size from being determinable on a retina mac. This is why the purple border is off.
Make sure you're on the latest NGUI. Open NGUITools.cs, go to line 1907 and add [System.NonSerialized] in front of it:
[System.NonSerialized] static Vector2 mGameSize = Vector2.one;