Why has the Supreme Court repeatedly ruled that flag burning is a protected form of political protest?

Government is not allowed to prohibit the action or expression of an idea [protected by the First Amendment] simply because the public or society finds it offensive. Flag burning to this day is an ongoing argument. There have been two cases: United States v. Eichman & Texas v. Johnson. Both cases were overturned. Certain "interest groups" and various government officials still fight to prohibit flag burning.