Of course they were.
Why shouldn't the situation be rectified, if possible, to give dignity back to the convicted's family?
OK, what if the guy is guilty? Some president gives a posthumous pardon under pressure from a convict's family and it's discovered later that the guy was guilty as sin and a monster. What then?
It's not like it takes a lot of effort.
You are exactly right. It takes nothing. It means nothing. It is an empty, useless gesture. It trivializes the awesome power of presidential pardons and I would much rather have the president overturn real injustice than give his stamp of approval to a family who is, apparently, not sufficiently convinced of their dead relative's innocence that they need an official imprimatur.
But it has become quite clear that Johnson was not a monster and that he was the clear vicim of racism.