Edited to read: http://latimesblogs.latimes.com/jacketcopy/2009/04/amazon-responds-to-adult-queries-blames-a-glitch.html
According to the LA Times it was all just a "glitch" but Mark Probst made an inquiry. The response he got didn't sound like a glitch. It is as follows:
In consideration of our entire customer base, we exclude "adult" material from appearing in some searches and best seller lists. Since these lists are generated using sales ranks, adult materials must also be excluded from that feature.
Hence, if you have further questions, kindly write back to us.
See also: http://lisybabe.blogspot.com/2009/04/amazonfail.html