Why would (or perhaps should they) care about PC users in general? They care about, and have an obligation (the kepler degradation is another issue) toward their own customers but what and why are they obligated to do for AMD and Intel users?
I feel this question needs to be answered first before any meaningful discussion can be made.
The answer is not: "Because its good for PC gaming." Nvidia is a business run by shareholders whom they have an obligation to turn a profit for. People are confusing what they want with Nvidia is obligated to give them (entitlement generation).
Personally I feel that the discussion must go toward and be applied to other examples to analyze strengths and weaknesses of the arguments.
Look at apple for instance. Want Final Cut, aperture, iphoto or an up-to-date safari (or numerous other applications)? Buy a mac. Apple's not going to give your their software, their work for free. If you want that software the only alternative is to buy a mac. Of course this negatively impacts computing in general but I don't see a huge fuss about this kind of thing. Are they obligated to provide windows and linux versions of their propitiatory software to improve professional photo and video editing?
That's an extreme case and the point could be made that there is no third party that is ultimately selling the goods. Nvidia however, creates and licenses its closed libraries who ultimately have the choice about integrating them into their own software (knowing the strengths and limitations).
The difference between Mantle and Gameworks must also be discussed. Mantle is only available to developers who sign up and are approved by AMD. I am assuming there are a lot of NDA's involved with beta mantle access and I do not know the minutiae. Its similar to gameworks in that its closed and limited to AMD gpus (perhaps they relented and let intel in - and nvidia if nvidia wanted in). Lets not forget the 'openness' of mantle which was developed and v1.0 created without any input from intel (who were told "go away") to work solely on AMD GPUs. If AMD was so concerned about openness why tell intel to buzz off when intel inquired about mantle details (mantle was already in several games at the point so a lot of the groundwork must have already been done - surely they could have worked with intel).
http://developer.amd.com/mantle/