basically because a racial slur tends to imply that the belief is still there. regardless if you say sorry or not. the point they now know is that Mel Gibson feels strongly enough to vocalize about it, drunk or not. if it wasn't inside to begin with it was random that he chose those words.
someone goes and gets drunk and decides to burn a cross...it wasn't a random idea. These are thoughts we take with us that manifest when inhibitions are low. if he truly felt anti-semitic that whole slurry of insults wouldnt have come out of his mouth. he would have chosen other words.