Why need to banned one way?
If you find dual diff easier to drive, use dual diff. If you find one way make you faster, why not use it? Let the player choose what is best for them. Banned the one-way will only limited the choice of the racer.
If the race authority get anal about one-way, they should also control the tire and motor. Let everybody use the same tire and motor. (or even the same chasis, and all these stuff is to be 'selected' by the authority)
I think it is a business issue rather than fairness issue.