As you yourself said, HBO had every right to do as they pleased. There is nothing to "defend" as they offered numerous opportunties to view an event that otherwise would have been closed to all.

I realize you have a different opinion on what they should have done, but I think we both agree that it was an incredible experience to have Pete Seeger appear at the event and sing the song.

It should also be noted that between the broadcast and the assembled crowd, more people probably heard those lyrics than ever before. That is what is important.