You seem to be intentionally avoided the key point, which I already discussed. You said, "If I am not mistaken they probably created a vendor specific subroutine in Dirt showdown" which is false. Now you say, "there may be a shader subroutine specifically suited for AMD" which was not done by design, because the coding was started or even completed before they had any chance to run it on Kepler.
There is nothing wrong with leveraging the hardware as much as possible, no one is saying that. Where it all goes south is when you intentionally make sure your competitors hardware runs poorly, or has visuals completely missing. Or disables a feature outright just because the system has a competitors card installed. See the difference?