The problem is that illegal drug use is disqualifying for a US visa, and Harry wrote all about his drug use in Spare. The Heritage Foundation made a Freedom of Information Act request to see his visa application. The government doesn't want to let them see the application and is trying to argue that Harry's admission in his book isn't evidence of actual drug use, which the US legal media is having a big chortle about, saying yes, precedent says that published statements which were asserted to be factual at the time can be used as evidence.
The judge has indicated that he will look at Harry's visa application and make a decision. If it is shown that Harry said on his visa application that he had not taken illegal drugs, then he lied on official documents and is subject to deportation, American wife and child notwithstanding.