Part of the challenge with Kinect is that if taken at face value - i.e. no controller whatsoever - then all input has to come from your movements. For most developers that clearly leans them towards 'on rails' type stuff as it's just easier to do.
I see no reason Kinect couldn't support 'off rails' but it'll need some developers to really think about how to implement a gesture based way of moving around in some fashion.
The Wii and Move offer a controller to allow for traditional movement, so its much easier for developers to consider how to get 'off rails' for those consoles... but Kinect represents a different challenge and one that is going to take some people to get their lateral thinking caps on to solve.
BTW I have no idea why anyone who wants to play a driving game would use Kinect (or Move or even the Wiimote on its own) as the input. That's just nuts and so wrong for that activity.
Try to be reasonable... its easier than you think...
