A quick glance over the YenPress and JNovelClub LN titles Amazon US has banned so far, suggests they are targeting those which have nudity of young girls in the color illustrations (often available in free previews). This is just an assumption, from checking the LN previews of No Game No Life, How Not to Summon a Demon Lord, The Greatest Magicmaster's Retirement Plan, I Shall Survive Using Potions, Clockwork Planet, Grimgar of Fantasy and Ash, and Infinite Stratos on Bookwalker.jp. Surprisingly enough they do all have at least one color image with partially hidden full nudity (not necessarily even sexualized, just nude) in at least one LN volume. This common trend could obviously be broken as they start banning more titles, if that isn't the actual reason. It would become rather worrying if they are banning over something more broad than that.