Sadly, you generally don't purchase a game, but a license to play one in some form. I think the exact limits of their control have not yet been tested in court, but I don't think NVIDIA wants to be the one testing it.

And any way, NVIDIA is in no position to challenge the games industry on such things - they are very much in bed with them and would lose enormously if games suddenly all went for a big 'best on AMD' logo.