Burning the flag is dangerous. People have set themselves on fire doing it.
And while watching them dance around and yell while burning gives me a warm glow inside it is kind of dangerous to other people.
So, how about we agree that fire is not speech and you make your point some other way?
That's what made the stupid law unconstitutional. If they had banned the ignition of ANYTHING during a public demonstration, the law probably would have stood up on First Amendment grounds (but should have been thrown out because it doesn't address any matter under Federal authority).
Once you ban the public burning of one flag but allow the public burning of any other flag, you've clearly violated the First Amendment.