I'd think that it's pretty obvious why they care. Game publishers don't like it when someone gives a promotional game download code to a friend, so they can install the game for free. Their suits probably consider that to be a "lost sale", where the person might have otherwise paid 50 bucks get the game themselves if they weren't given a download code.
Of course, most of the games that are bundled with video cards often aren't worth anywhere near their retail prices to purchase. The suits probably wouldn't want to admit to that in public, though.
I'd also imagine that Nvidia also doesn't like the idea of someone using the game download codes that they paid for being used on other systems with AMD and Intel graphics, but this probably doesn't bother them as much as the game publishers. They still got paid for the hardware that got purchased, anyway.