People commit crimes and they're allowed to re-educate themselves, in the criminal justice system, it's one thing we all want to see - people re-integrating into society, but that doesn't mean we prevent them from passing on their experiences to others, in fact we encourage it. The same doesn't seem to go for what things some people said or did in the past though, which was the nub of his programme. Some people want to continually punish them forever which seems harsh to most observers. "Times were different then" is not an excuse, it's a fact, otherwise society isn't evolving and hopefully improving, but there really was a time when blacking up didn't instantly raise horror in everyone's mind like it does - thankfully - today. And it's not that far back.
He plays the court jester on QI so is the very essence the butt of many jokes. Not that I want to defend him, his rants are outrageous and he will hopefully pay for them by losing work, as he should.