The United States has a strong legal framework to regulate freedom of speech. But that manly regulates what is a criminal offense (something that is an offence

The United States has a strong legal framework to regulate freedom of speech. But that manly regulates what is a criminal offense (something that is an offence that violates criminal law and can be punished by the state). However it do not necessarily protect anyone from sanctions from your employer. In this case it is his employer that has acted, not the state.