Congratulations on flying the flag today, Charles.  Well done!

Spitting on the flag, walking on it, or burning will incite many decent people to violence, especially those who have seen their good friends die to defend it.

Those currently fighting Daesh, the Taliban, and al-Qaeda are defending civilization against barbarians.

The Westboro Baptist Church's demonstrations at funerals may be protected speech, but it is extremely bad politics.  They alienate all decent people.  Similarly, when alt-Left creatures desecrate the flag it also alienates all decent people, including progressives like you & Peter.

Hence it is similar to yelling 'fire' in a crowded theatre because it is very likely to incite good people to violence. 
Imagine if you had just returned from a deployment in Syria and saw a bunch of characters burning a flag, you would be incensed to the point where you might well cause them considerable harm.

Under those circumstances, a jury probably would not convict.