There is another principle which protects employees from exercising a legal right, such as “the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.” Of course the employer could claim that the event in DC led to some illegal activity, but firing someone for supporting someone politically leads to a system like communist China, social media and all.
Should say “from being pusished for exercising a legal right”
Since what happened at the Capitol has been labeled a riot or an insurrection or domestic terrorism, take you pick, then everyone there has been tarred with that same broad brush. Companies will fire employees who participated and who were dumb enough to post on social media, and there's nothing illegal about it.