Screen.autorotateToPortrait = false;
Screen.autorotateToPortraitUpsideDown = false;
Screen.orientation = ScreenOrientation.AutoRotation;

Script above prevents screen rotation to "portait" and "portrait upside down" orientations, but when built on Win Store Apps, application can rotate to all four orientations.

Also, in editor's player settings, under "Resolution and Presentation" tab for Win Store Apps there is no option to choose default orientation or allowed orientations, compared with WP8 or iOS settings.