Honestly this doesn't surprise me one bit, even though GeForce NOW is quite a bit different from let's say Stadia. nVidia basically rents you a PC to play the games you own while Stadia actually sells them to you and also offering a pc. GFN doesn't actually get a slice of the profit of in game purchases or purchases as a whole (where stadia does) so it doesn't make any sense to prohibit GFN users from playing their games.
But do we remember the Twitch debacle? Activision wanted massive amounts of cash from twitch for the rights to their e-sports leagues. Of course Twitch declined. And now you can watch OWL and MDI on youtube lol. My point is, probably Activision wanted money from nVidia so they could provide AB games, and nVidia of course declined.
Activision Blizzard isn't doing anyone or themselves a favor with this though.