TressFX and Mantle have been open to Nvidia. They even hustled to get a patch up for tomb raider for Nvidia owners. AMD has been open doors.
What Nvidia is doing is perfectly fine as a corporation, but as a consumer and a part of the PC community (owner of both brands), this is not good. It looks like competition but it's really monopolizing to an extent.
At the end, the consumer will pay. Nvidia has the right to do what they are doing. The PC community has the right to be upset about it, especially when they see AMD willing to play ball.
For Mantle to work with Nvidia they would have to completely change their architecture. Never going to happen. AMD wasn't the one hustling for a TressFX patch for NV users, it was Crystal Dynamics. Neither company is innocent in this game.