One more thought ... flag-burning is inarticulate and pointless, like screaming and swearing instead of talking things out. Peaceful public communication is a fundamental right. Don't burn your flag on my lawn, but file a permit and do it peacefully downtown if that's the best you can do.
Besides, if you're going to lock people up for being inarticulate, inflammatory or offensive, I can think of a few people who wouldn't have a chance.
