This is the law he's been charged with breaking.
http://www.law.cornell.edu/uscode/18/1465.html
Production and transportation of obscene matters for sale or distribution
... sale or distribution of any obscene, lewd, lascivious, or filthy book, pamphlet, picture, film, paper, letter, writing, print, silhouette, drawing, figure, image, cast, phonograph recording, electrical transcription or other article capable of producing sound or any other matter of indecent or immoral character, shall be fined under this title or imprisoned not more than five years, or both.