Unless I missed it (I know I broke Slashdot rules and actually read the ruling) I dont think he had actually admitted to the crimes themselves, only that he did admit that he was in possession of the Encryption Key. Presumably they already had compelling evidence of the crime itself or they wouldn't have been investigating him in the first place.
From the Ruling:
On the day of his arrest, the defendant was interviewed by law enforcement officials after having
been advised of the Miranda rights. In response to questioning, he said that he had more than one
computer in his home. The defendant also informed the officials that "[e]verything is encrypted
and no one is going to get to it." In order to decrypt the information, he would have to "start the
program." The defendant said that he used encryption for privacy purposes, and that when law
enforcement officials asked him about the type of encryption used, they essentially were asking for
the defendant's help in putting him in jail. The defendant reiterated that he was able to decrypt
the computers, but he refused to divulge any further information that would enable a forensic
search.