I doubt Joe Kennedy was a Nazi sympathizer, though I do not know for certain. He was the US ambassador to Great Britain; somehow the two don't go hand in hand. That he was an opportunist, manipulator and crook there is no doubt. Indeed, people who were alive at the time said that FDR appointed Kennedy to that post to save him from further investigation and possible prosecution for his bootlegging and stock manipulation activities. That Kennedy was a big time contributor to FDR campaigns didn't hurt.
I would actually forgive or even ignore Schwarzenneger's father's supposed activities. Other than the bigwigs in Germany, the average person had some serious considerations. First and foremost, if you have a choice between the Nazis (whose track record of improving Germany wasn't too bad through the mid 1930s, let's be honest) and the Russians - and those are your only choices, no ringers or space aliens to help you, what are you going to do? Remember, your answer affects your family. Second, just as with many places here in the US that time, you had to be of the right party to get certain jobs. Schwarzenneger's father had to deal with what was at the time, not some ideal. I wonder what the Keyboard commandos would do under similar circumstances. He did what he did to survive, Kennedy actively did whatever he may have done as an opportunist, and out of greed.