Here's the first attempt I've seen to defend Amazon against the double standard charge, and it looks to me to be a plausible explanation. Anthony Rickey says the reviews on this book have skyrocketed while sales haven't (at least from Amazon), and he thinks Amazon simply doesn't want to have to slog through all of them to figure out which ones violate the policies. The best they can do is do a keyword search for obscenties and just ban those reviews. I can understand that. He adds that they should have been honest about their reason if that's what it really is, and I agree, though I can imagine that they might have feared something like that happening on any book unrelated to politics that people want to get combative over. At least this way they hope not to have to do this with anything after the election is over. I don't think that's a good reason to deceive, but it's a plausible explanation.
Update: I just looked at my referrals, and it seems someone on Amazon's own network has been reading my site, coming in from the same site where I found the above argument. I just tried to go there again, and it was busy, so I guess this site is getting a lot of action right now. I went back and checked the Unfit for Command policy, and it's still worded as it was despite all the discussion.
Update 2: Here's a borderline alarmist criticism of doing things the way they did. It's a worthwhile worry for Amazon's sake and for those who will want to use Amazon as a resource.