It's all about whether you have a reasonable expectation of privacy, and whether the show is worried about privacy tort litigation.
For the poster of this thread, chances are any judge would rule that you had no expectation of privacy if you were singing in a choir, knowing that people would make home videos. Whether it was somebody's family or the entire nation seeing you on TV you should have known you'd be filmed.
Even if you could prove that you had an expectation of privacy, the TV network or whoever would make sure it cost you more than you can afford to litigate.
For TV shows, they block out people's faces if they can't get them to sign a consent form to show them, but even then it will only be in places where those people had a reasonable expectation that they wouldn't be randomly filmed.