This is the problem because 99% of people are effectively dependent on their employers, and there's huge power imbalance between employers and employees.
In Europe there's a specific list of reasons which can result in work contract termination. This means that the employer cannot fire me just because I posted on Facebook "vote for party X for better future". This means that I feel free to post this because I know nobody will ruin my life over that.
In most US states there's a specific list of reasons why people cannot be fired - which means they can be fired for all other reasons, and "posting on Facebook that you support given party" does not belong to that list, which means that you can be fired for that, which effectively makes you think twice before you say things publicly.
I really don't think that "you don't go to jail but you have your life ruined by being fired and becoming unemployable" is such a fantastic deal and an example of freedom of speech.
