Apple did not point out which contents is illegal in China, but we think maybe is the three book from China writer Wang lixiong (the three books at bottom).
Some friends tell me this is not first case, but I still think this maybe because Apple use CDN bring their content to China, so some Censorship, coordinate is part of deal.
(still can be download in other country, but China.)
FT:Apple bars China app for ‘illegal’ content
TNW:Apple accused of kowtowing to China as it follows recent apology by banning app for ‘illegal’ content
CNET:Apple bans Chinese bookstore app over 'illegal content'
Yahoo:Are iOS apps the next victims in China’s war on Apple?