Well, Nvidia does not sell video cards and we are not their customers. Such features are advertised but so too are the requirements stated and it is up to the AIB whether they include a decoder. It is understandable that they generally do not however due to cutthroat competition (and thus slim margins), relatively insignificant user demand for the function and the relatively high cost of licensing.
To their credit, ATI does have a deal with Cyberlink to offer their decoder for $15 whereas the Nvidia "deal" is still $50 for a complete PowerDVD 6, versus the normal upgrade from a 6 OEM version (free with virtually every card and drive) to 7 Deluxe being only $30.
So, in some ways the H.264 situation is not unlike that with MPEG-2 a decade ago when there were no "free" decoders available. Microsoft provided a "DVD player" with Windows but not a decoder. Legally, royalties are still due so either the provider is paying them or breaking the law or in the case of libmpeg2, libavcodec and such are skirting the law by not offering compiled software directly but leaving that up to others and in any case not selling it.
But as said for H.264 whether legal Apple or quasi-legal libavcodec, none sport DxVA. At least be thankful there are free decoders even if lacking acceleration. BTW, the "free" MPEG-2 decoders likewise lack DxVA so by buying a decoder/player package you get both and while MPEG-2 SD content (olde timey DVD) hardly requires acceleration these days, it is nice to have for the HD variety.