Current laws do not provide the ability of most Americans, especially those that work for private employers, to participate in free speech.
Employers may use speech outside of work hours to reward or retaliate against employees, be it a t-shirt they wear, a tweet, or even having a beer. While there are cases where speech may reasonably be limited, those should be the exception. ex: Someone that supports a presidential candidate with a bumper sticker on their car should not be able to be fired by their employer (recorded case).
This is equally problematic for employers. Allowing an employer to reward or sanction free speech can essentially force them to act lest they be perceived as being being supportive of it. This is especially true for what is viewed as a divisive issue.