I believe that technically he was convicted on obscenity and the reason he was utterly screwed is that he had the magazines sent through the mail. The supreme court rulings on the matter do not make lolicon legal, what they did was say that lolicon/virtual depictions could not be considered child pornography. This however doesn't prevent it from being illegal as obscenity. The thing with obscenity is that possesing it is not a crime but distribution and transportation is.
As articulated in several sections of 18 USC Chapter 71, the Supreme Court has ruled that it is constitutional to legally limit the sale, transport for personal use or other transmission of obscenity. However, it has ruled unconstitutional the passing of law concerning personal possession of obscenity per se. Federal obscenity laws at present apply to inter-state and foreign obscenity issues such as distribution; intrastate issues are for the most part still governed by state law. "Obscene articles... are generally prohibited entry" to the United States by U.S. Customs and Border Protection.[4]
Now I generally think all obscenity law is bullshit but that is what he got nailed on, not child pornography.