Pardoning Jack Johnson does a number of things:
1. It increases Trump's stock in the minds of black men who started gravitating toward him with the Kanye West kerfuffle.
2. It targets sports fans. It's noted that Sylvester Stallone and Lennox Lewis were on hand at the pardoning ceremony--showing racial unity, rather than division as was common with Obama.
3. It marginalizes Obama's legacy among blacks, as Obama never bothered to address some of these historic wrongs, preferring to use his community organizer tactics to divide people instead.
I suspect, in time, Trump will pardon Edward Snowden; however, it is too soon to do that right now. So far Trump pardons include:
1. Joe Arpaio: Basically, "fuck you Obama and Holder for siding with illegal aliens over law abiding citizens and law enforcement"
2. Scooter Libby: Basically, "fuck you Comey and Patrick Fitzgerald for convicting someone who didn't leak any CIA names to the press"
3. Kristian Mark Saucier: Basically, "fuck you Obama, Holder, Comey, Hillary, et. al for heartless and ruthless double standards."
4. Joe Jackson: Basically, "Look! Obama really wasn't for black people. His role was to bail out Wall Street billionaires at the expense of the middle class."
-- Joe Biden