How, exactly, can Nvidia make games run poorly on other hardware? They don't write the games. Both AMD and Nvidia have extensive outreach programs to developers and make engineers available to game studios
That is true, but nVidia's outreach engineers have a history of checking code that regresses performance on competitor hardware. See what this Value developer has to say about "Vendor A": Vendor A is also jokingly known as the "Graphics Mafia". Be very careful if a dev from Vendor A gets embedded into your team. These guys are serious business.
How can using certain benchmarks (as you suggest) make games run slower on other hardware?
Thats not what I'm suggesting. I am suggesting that nVidia has a history of being dishonest which thier performance benchmarks. The worst case by far is during the GeForce FX era when they were caught a driver that detected it 3DMark 2001 and then only rendered content that was visible to the camera instead of the whole frame to boost thier benchmark scores. That was a while ago and I've been unable to find the original story on it.
AMD fanboy much?
Not at all, my desktop currently has a GeForce 570 installed. When I bought it nVidia clearly held the performance crown. That said, I really don't like the unethical business practices and I think I might not buy them again.