You see many people see the American flag as not being the flag of the government of the United States of America, but as being a symbol of the people of the United States. So when you burn it you are not just attacking the government, but attacking the American people as a whole. That makes you a traitor in many people's eyes. It goes further than that. Many see the American flag as symbolizing a certain set of ideas espoused during the American Revolutionary War. They link it to ideas as justice, freedom, human rights, and such documents as the Declaration of Independence and the United States of America Constitution. It is in the original Pledge of Allegiance:
I pledge allegiance to the Flag of the United States of America, and to the Republic for which it stands, one Nation, indivisible, with liberty and justice for all.It does not say "I pledge allegiance to the Flag of the Government of the United States of America..." but "to the Flag of the United States of America" i.e. all of us that are American citizens, to all of us that are a part of this country. Further, it says "with liberty and justice for all." So there in the very pledge it says the flag represents the people and stands for certain ideas. If the government no longer represents the people, or no longer represents the ideas that this country was founded on, then the government's flag is no longer the American flag. The American flag, I feel represents the American people, human rights, freedom, and justice.
So next time you decide to destroy a symbol of a government you see as repressing you, do not choose the American flag. Instead, loudly proclaim that the American flag does not represent tyrants, bigots, or oppressors. State that it is your symbol, and you plan to take it back from those whose ideas conflict with those the American flag represents. Find another symbol of the government to take your frustrations out on. Burn politicians in effigy, but I beg you do not desecrate the our flag, the flag of the American people.