It's like Moody's. More political than educational. Downgrading the US may be fair, but the main purpose it serves is driving clicks. More to the point, the fact they downgraded the US shouldn't tell the average HN reader anything they don't already know. Contrast to say, S&P.
Edit: By the way, look at the way the top of the "Flawed Democracies" cluster under 8.00! I can totally believe some tweaking went on here.
Isn't it quite reasonable that it's based on past performance rather than being a guess at what might happen in the future?