I agree. You KNOW that if a white guy started calling some black guy a "n*gger" and got punched out, the "fighting words" defense would be upheld in any resulting lawsuit.
The only way I see of halting flag-burning is for the offended American patriots nearby to start pounding the crap out of the burners. Maybe the government cannot arrest the flag burner due to 1st Amendment, but pissed off Americans sure as hell can do something about it. Problem is, obviously, no one wants to get involved in a legal hassle. It's easier to just walk away.
If the Leftist Courts fine people so badly they cannot continue in business for NOT baking a cake, Yeah, you can expect hell to pay for punching one of the flag burners out. That's how the Left rolls. Just defending against a lawsuit can financially destroy the average person, but you can bet your last bite of brisket that they'll push for you in jail, too.
Know them for what they are, express your opinion, too, even to the point of extinguishing the flag, and taking it. Few police would arrest you for that. But don't touch the protesters short of self-defense.