Does anyone know the "technicality"?

He was not prosecuted because the wiretaps used to gather information against him were done without a search warrant. Therefore any information including the evidence against him was fruit of the poisons tree and could not be used against him in court. In short the prosecution really screwed up and made a huge mistake. Because the wiretaps were no good, they had nothing to corroborate the eye witness and informant against Ayers and could not prosecute him.