Patrick advances a pretty plausible theory for how Amazon might have come to do this astoundingly stupid, offensive thing.
Update: Right on the money: a cataloging error caused 50,000 titles to be misclassified as "adult"
(1) Sometime in the middle-distance past--maybe a couple of months ago, maybe a year, it doesn't matter--somebody decided that it would be a good idea to make sure that works of straight-out pornography (or, for that matter, sex toys) didn't inadvertently show up as the top result for innocuous search queries. (The many ways that this could happen are left as an exercise for Making Light's commentariat.) A policy was promulgated that "adult" items would be removed from the sales rankings and thus rendered invisible to general search.Amazon's very bad day
(2) Sometime more recently, an entirely different group of people were given the task of deciding what things for sale on Amazon should be tagged "adult," but in the journey from one department to another, and from one level of the hierarchy to another, the directive mutated from "let's discreetly unrank the really raunchy stuff" to "we'd better be careful to put an 'adult' tag on anything that could imaginably offend anyone." Indeed, as Teresa pointed out, it's entirely possible that someone used a canned list of "adult" titles supplied from outside, something analogous to the lists of URLs sold by "net nanny" outfits, which would account for the newly-unranked status of works like Lady Chatterley's Lover. (As one net commenter observed, "What is this, 1928?")