I agree with the press freedom points, but there’s another angle here. Barr can’t (without it looking really bad) cancel the previous charges, but by adding these on, he reduces the chances of Assange being extradited.
The missing link in the collusion investigation is evidence that Trump himself knowingly colluded with a hostile foreign power. Assange, as the cutout between the Russians and the Trump campaign, could have that piece of the puzzle. So having Assange extradited is not in Trump’s best interest.
Ergo, this is an attempt to prevent Assange from being extradited. And getting the press to help prevent it is a bit of evil cleverness on Barr’s part.