You raise an interesting point...
I think maybe part of the clue could be in the last line of the "explanation of ratings offeren on the BBB website:
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F We strongly question the company’s reliability for reasons such as that they have failed to respond to complaints, their advertising is grossly misleading, they are not in compliance with the law’s licensing or registration requirements, their complaints contain especially serious allegations, or the company’s industry is known for its fraudulent business practices.(emphasis added by me)
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I would not be a bit surprised if webhosting companies' reputations in general might be part of the evaluation.
It's also interesting to me that the BBB record was "created" over 5 1/2 years ago...and there is no way for me to tell from the website when the 7 listed "unanswered" compaints might have been made.
Frankly, I'm entirely encouraged by the fact that a company that has grown to the size of DreamHost has only 7 compaints listed with the BBB, given the often clueless expectations and general ignorance of webhosting evidenced by many customers of "low-cost" hosting. I would have thought it would be normal for DH to have had complaint numbers several orders of magnitude higher than that!
My guess is that the combination of "not being a member" of the BBB, and "not answering" the 7 complaints is the reason for the "F"....they don't participate.
7 BBB complaints in 5 years? That, by itself, is a great recommendation for DH, IMHO.