Why do joypad settings affect mouse movement?

After playing this game a few times i was getting frustrated with mouse aiming. after several tests it was apparent that vertical sensitivity was a lot higher than horizontal sensitivity. also there was an inside and outside deadzone which causes unusual mouse aiming.
I have seen loads of people saying the same thing and it turns out that if you plug a controller in and go to controller settings and click on aim sensitivity it seperates into horizontal and vertical sensitivity, and lo and behold vertical sen is on 10 and horizontal is on 5 so i changed them both to 10 then went to deadzones and chnaged them so no outside deadzone and no inside deadzoneā€¦could not turn accel all the way off though.
Now mouse aiming works correctly.
So i ask this question. Why the hell have you made controller settings affect mouse and keyboard settings?
Anyone else out there please try this it will make mouse movement 100 times better.