Personally, I'm as big a critic of Blizzard as anyone when I think that there's something to complain about, but this rating hardly seems justified -- even if the BBB has received tens of thousands of complaints, that's still just a small portion of the playerbase. And despite the occasional downtime and various class nerfs, they hardly deserve an F rating, especially when a company that many people really do have issues with is riding along with an A rating. The BBB page also says that Blizzard's mass bannings have been a factor in many complaints -- there is probably no distinction made (or that can be made) in terms of complaints between people who have broken the ToS and people who have not.
At any rate, even if the F rating is there, it obviously has very little effect on Blizzard's business -- how many of you ran to check the rating before you decided to subscribe to World of Warcraft? It seems like a few customers (who may or may not have broken the rules to begin with) have ruined Blizzard's reputation with the BBB, but it's fairly apparent that the BBB doesn't hold much sway among Blizzard's customers anyway.