To understand Kennedy's book, perhaps it would be easiest to start with the final example of his eight Profiles in order to see his opinion at full volume. It's at 'full volume' because the issue in question is the justice (or injustice) of the Nuremberg Trials. That means that it is the ultimate version of the social courage that Kennedy is trying to elevate, because if a person is willing to speak out in defense of Nazis, they certainly are brave, since most people wanted the Nazis to all be killed without a fair trial.
But that was exactly Robert Taft's concern with the Nazi trials. Even though it cost him his chances at becoming president (it was likely that he would have been a successful candidate otherwise), he still voiced his opinion. He believed that because the Nuremberg Trials were convicting Nazis for rules they had just made up, that it constituted ex post facto legality, which seems unjust.
However, Kennedy does not praise Taft for his opinion about the Nuremberg Trials. He praises him for his bravery, because it was an ultimate act of bravery and self-sacrifice to speak honestly about his convictions, even when it meant he would be easily smeared as a Nazi sympathizer.
Kennedy's version of justice is when a brave person can stand up and argue persuasively against the public opinion, understanding all the while that such an act of courage may bring real political backlash.