All the government did was prove that the Apple content was Hunter's and it was obtained legally by them, since the laptop was "abandoned".That's it, and that's all they needed to do. This case doesn't care about any of the other files on the hard drive, they just want pictures of Hunter using drugs when he owned or submitted the permit for the gun. I would think they are probably going to see if they can make a case that he had intent to distribute as well.
This is the similar to how they took down FPSRussia. Find him with some drugs and send them to jail. Literally everyone who smokes weed and owns a gun is guilty of the same crime as Hunter in this case, and that's a lot of people