As I have already written, I suggest it is a priori plausible that fire is "burned" onto our neurons (pun intended) as the ultimate symbol of mindless destruction. Fire destroys, and it does so in a chaotic and uncontrollable manner. It is hard to think of something that would be a better symbol to incite the inner "irrational man" that seeks to wreak destruction as the means to solve a problem.
I am puzzled that people do not see this - I suggest it is no coincidence that flag-burning is often, not always I concede but nevertheless often, accompanied by frenzied, out-of-control chants for the death of some group of people.
Fire is so primal, and so destructive and chaotic that I believe its use in symbolic burnings presents a risk for incitement to violence that is higher than for other forms of "speech".