Common sense
If someone burns a flag, they haven't damaged me in the slightest. Let 'em burn, I don't care. If someone shoots me in the foot, now they have done real damage. Now we have a real dispute. Burn a flag, you've said you don't like whatever the flag stands for. Big Whoops. Bloody my nose and we are going to have a tussle.
Am I going to get upset because you burn my flag? I have better things to do, even if you don't.