Burning the flag is NOT political speech. Political speech is related to constructive debate on important issues, and which person or party represents the best course of action to address those issues. It pertains to electing representatives to our governing body (legislature, executive officers), that will work at the direction of their constituents.
Burning the flag is destructive, belligerent and antagonistic. It serves no constructive purpose, and does not warrant protection, as it does not serve the common good of the people; inciting to riot is a violation of law, and flag burning is associated with riotous actions.