Isn't flag burning a very powerful means of expressing a message?
So is threatening the President or joking that you have a bomb in an airport, but we don't allow every powerful means of expression in our society. Let's not romanticize the issue too much. In our society you will still have the right to say anything you want to about the flag or our government, you will just be restricted from physically harming the flag with impunity. Most people do not think that this places a burden on anyone's ability to communicate whatsoever.