If thier conduct is not solely expressive. You said it your self in that quote, burning a flag is not, and can not be made illegal if it is done in protest. This was done in protest. In a traditional public forum, where you have the right to burn a flag. In protest. The Supremacy Clause states that no local or state government can make a law that would take away or circumvent your rights. Buring a flag, in a park (traditional public forum), in protest is not, and can not, be made illegal unless the constitution says so.