About in air control

I am confused with the logic concerning in air /(!grounded) rules. Why does the editor stop allowing you to control a player when you're not grounded? I understand the need for realism (normally when falling you can't propell yourself forward or backward), but I feel that that is something I should be able to chose to not allow by script...

Anyway, how do I allow in air movement? I saw the script on the wiki site but I don't understand it. How do I simply

'continue to propel the player normally when (!grounded)' ??

Isn't there simply someway to allow transform.forward/ controller.SimpleMove.forward when in the air?

The general movement script I am working with is

var controller : CharacterController = GetComponent(CharacterController);

var forward = transform.TransformDirection(Vector3.forward);

var curSpeed = speed * Input.GetAxis ("Vertical");

transform.Rotate(0, Input.GetAxis ("Horizontal") * rotateSpeed, 8);

controller.SimpleMove(forward * curSpeed);

I am working with a character controller. Thanks for any help.