Based on what I've seen, there's a number of different classes of NDA violation. There's a huge difference between somebody taking screenshots and putting them on their desktop pattern, and people selling hack kits, trying to start their own servers, ect. It would not suprise me if people haven't tried to sell the beta.
I suspect the ban on benchmarks is more about message control, keeping ignoramuses from making ridiculous statements. Some pictures and word of mouth actually helps more than it hurts.
Real life example. I know a guy who finagled himself some beta time. Runs a dual 8800, but only two gigs of ram, which is, of course, loaded down with 50 widgets and the spyware that comes with his wrestling porn addiction.
He's has a ludicrously skewed idea of what the expected hardware requirements are.
I'm betting the letter was written by a harried bee eater, who's inbox is filled with 250 thousand requests for invites, and another half million requests for tech help on how to pirate the game, and considering the level of ass hattery they're dealing with, I don't blame them.