Because in Japan it does not allow the camera shutter sound to be turned off by any legal means.
So how to make it silent? because you will use it and you are not in Japan.
The solution :
You can use the small application named: Mute Camera.
Go to Play Store, search the App using keyword: mute camera.
I recommend you to choose Mute Camere Free from HDM Dev Team.
Install it and then you are ready to use the camera with silent mode.