You are confusing two separate things called "motion". CMMotionManager is used to access the sensors, such as the gyroscope and accelerometer, that report how a user physically moves a device. It has nothing to do with the motion effects, like UIMotionEffect objects, that are used in animating views.
The deviceMotionActive method merely indicates whether your app is currently registered for receiving motion updates from CoreMotion. This will only be true if your app has called one of the CMMotionManager startXXXUpdate methods. Again, it has nothing to do with user settings or UIMotionEffect objects.
UPDATE: As John mentions in the comments, there appears to be an API for this in iOS 8: See stackoverflow.com/a/25453082/2904769 .