It's the First Amendment. It may be the one amendment that is most known by casual and disinterested people that don't often think of the Bill of Rights.
There is no more American ideal or precept. The government may not abridge the right of free speech.
Over the years, there have been several carve outs that have become generally accepted. It's been agreed that this Constitutional protection doesn't allow for shouting "Fire!" in a crowded theater. In the last few decades, "hate speech", has lost it's protection, even when it's not always easy to know what exactly constitutes hate. Law isn't always nice and tidy.
Commercial speech, the ads one sees on TV have limited protection. Lying in the service of selling used to be a prohibited act, but our Supreme Court has conferred a veritable personhood on corporations, so it may soon be that corporations will have the same free speech rights as living people. We'll have to see how that develops over time.
In spite of these several exceptions, you might think that citizens of the U.S.A. would be proud recipients and champions of this constitutional protection. One might expect that this quintessential American ideal would be a guiding light for this country, and held up as a model for this society to function.
Well, it ain't the case.
While corporations are achieving parity with flesh and blood people in "civil rights", those same flesh and blooders don't really have a First Amendment that can protect them. Truth tellers have few rights when it comes to whistle blowing. When criminal or embarrassing behavior is revealed by whistle blowers, the full weight of prosecutorial action is often unleashed by the government against the revealers of truth. Whistle blower protection laws have so many exceptions so as to render them next to useless. So much for American ideals and principles.
This has happened when government secrets are revealed, but the same lack of protection exists when people post embarrassing things on their Facebook accounts, but the punishment is meted out not by government, but by companies. Potential employers now routinely screen employee candidates social media accounts to determine their "employment suitability". Companies have fired people when some unapproved items are posted by individuals. For years, unions have been fighting for their rights, but union organizers are so often fired for their economic speech. The First Amendment offers no help there. No allowance here for American ideals and principles.
The First Amendment only protects political speech. Most people in this country are blissfully apolitical. The First Amendment makes for a rousing day in civics classes across the nation, but it has no effect on the daily lives of millions of people. Their employment, however, impacts their lives intimately on a daily basis, and there is no protection for any speech deemed inappropriate by employers. The tyranny of King George was cast off by a revolution. The tyranny of the economic sphere is much more powerful, and renders it's subjects thoroughly powerless. No allowance here for American ideals and principles, either.
As our rigged society bifurcates into the wealthy and everyone else, it's not just the money that is being funnelled up to the one percent. Rights of redress are also being lost by the ninety nine percent as the corporations are allowed to buy the political process and are allowed to impose their own rules of social engagement for everyone else. Still no allowance for American ideals and principles.
Welcome back to the Gilded Age.