Flag is a symbol, remember?
Destroying your own flag would be like printing a big letter "S" and burning it. You wouldn't do harm to the alphabet by destroying this "S" nor would you harm any words that used the letter. You would do no harm to the English language, yet if you did the same thing with a flag, people would erupt in violence.
If a man in North Korea were arrested for stomping on a newspaper photo of Kim Jong Il, we would condemn his arrest as a form of political repression. However, if a man in Denver were arrested for stomping on a U.S. flag that he purchased at Wal-Mart, many of us would not recognize it as political repression. Many would say it's OK to arrest a man for harming a flag, because we've forgotten that the flag is a symbol.