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According to Nofsinger the main reason behind the removal of support for the device was due to the fact the game was built from the ground up for use of the Wii Remote alone and that Wii Motion Plus really didn't add much to the experience. Eric also said that when the peripheral was included it didn't have many uses while in gameplay and that it felt "tacked on".
The removal of support for Wii Motion Plus is not exactly a surprise. It has been proven in the past that First Person Shooters on the Wii can have great controls with the standard Wii Remote setup. The new peripheral does increase motion detection to a 1:1 ratio but it isn't a necessity for the FPS genre.
Source: MyWii