With a few narrow exceptions the Constitution and the federal laws derived from it only protect a person’s right to expression from government interference, not from the restrictions a private employer may impose, lawyers say.
[...]
While federal statutes such as the 1964 Civil Rights Act, the Age Discrimination in Employment Act, the Americans with Disabilities Act, the Genetic Information Nondiscrimination Act, and the National Labor Relations Act limit companies’ rights to fire or hire workers and prevent them from joining unions, these restrictions are based only on race, religion, ethnicity, sex, age, and a few other protected categories. Political opinion isn’t protected by any of these statutes.
“The First Amendment applies only to employees of the government in certain situations, and all citizens when they’re confronted by the government,” says Mark Trapp, an employment lawyer with Epstein Baker Green in Chicago. “In other words, freedom to speak your mind doesn’t really exist in work spaces.”
Source:
http://www.businessweek.com/articles/2012-08-03/where-free-speech-goes-to-die-the-workplace