There are no laws prohibiting employers from disciplining employees who PM other employees.
Workplace retaliation is only illegal in narrow and specific contexts (such as retaliating against employees who file wage claims with the DOL, or retaliating against employees who state they don't wish to be sexually harassed, and so on). This isn't one of those contexts.
In the legal sense, freedom of speech is about giving you the right to state negative opinions about the government without fear of the government retaliating against you for expressing said negative opinions.
Finally, if your real name is Michael Jones, please note that anyone - including your current employer and future prospective employers - can find everything you post on these forums by performing a simple google search.

