The flag of the United States of America is defined in the government legal code as a symbol for the government. It is worn by government officials who enforce government policies and laws. It flies over government buildings. It is therefore entirely reasonable to say that the flag represents the government.
Judging by the vitriolic partisanship of voters and the low voter turnout rates, not to mention Congressional approval ratings, it's safe to say that there is no argument to be made for government representing the general population. Politicians are notorious for their lies, cronyism, deception, and corruption. The military enacts the foreign - and sometimes domestic - policies of these selfsame politicians. Government laws are frequently unjust violations of the very rights governments claim to protect and defend, making the concept of "law enforcement" into a fundamentally immoral profession. Why then should symbols of government be respected?
Does the flag represent certain values, morals, or principles beyond the government, though? True, one can argue that the meaning of any given symbol is subjective, but if that's the argument, how can it be objectively wrong for some people to perceive those symbols as repugnant and treat them with disdain? If the flag represents freedom and individual rights, how are these principles harmed by the exercise of those rights in destroying the symbol? Even if the flag does somehow represent the noble transcendent concept of freedom, each specific physical representation of that flag is a unique piece of physical property, and property rights as a principle of freedom applies.
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Why do so many people treat the flag with religious veneration? It is a piece of cloth with a pattern on it. That is all. Someone burning the flag is no more an assault upon others than is burning any other piece of cloth. Trump tweeted that burning a flag should be considered a crime. Hillary actually proposed a law that would criminalize it. Both of these individuals are corrupt, abusive monsters seeking political power over the lives of others to plunder the productive economy and threaten violence against peaceful people. If the flag can ever represent them or their principles, the only rational response is to burn it. Don't succumb to patriotic idolatry.