Arguing against the posthumous pardon of Jack Johnson feels like voting against giving the Congressional Gold Medal to Mother Theresa, but I'll offer a contrarian take.
Of course, the injustice in Johnson's case is undeniable. His conviction was wrong and racist and all sorts of wow-America-was-awful-back-then.
But what does a posthumous pardon accomplish? It doesn't clear the slate for Jack Johnson, who's not around to benefit. It clears the slate of the entity that convicted him, and which does still exist: the United States government.
I feel the same across the board about posthumous pardons of historical figures. It feels less like correcting an injustice, and more like the government congratulating itself for becoming more enlightened and officially whitewashing its own record.
The fact of Jack Johnson's conviction shouldn't be expunged, it should stand as a stain on the government which produced it. The same goes for the long litany of wrongful convictions and miscarries of justice which this government has on its historical record, most of which will never be famous enough to have a celebrity pressure campaign on their behalf.
There's also the argument long advanced by the Office of the Pardon Attorney, which is that posthumous pardons and reviewing requests for them is an unwelcome distraction from the real important function of the power: to correct injustices that are still ongoing in the here and now, for people who are still alive. While Trump basked in the glow of his purely symbolic action, there are untold thousands of people still in prison, or suffering from the disabilities of being a convicted felon, and who are deserving of clemency.
Posthumous pardons are about the government trying to absolve itself of what it did, and present-day politicians trying to take credit for it. The presidency is a temporal institution, not the dispenser of cosmic justice or decider of the fate of immortal souls.