If I am an employer and I find out through an off-duty activity that an employee is a racist and I don't want to risk having a discrimination incident, I'm getting rid of that person.
Back in the 70s, a man was watching television one night and saw an employee holding up an "Impeach Nixon" sign. The man was a Nixon fan, so he fired the employee the next morning.
There was a study some years ago (pre-Trump) showing that liberals were more likely to block conservative friends on Facebook for political content than vice-versa. Conservatives praised the research because it seemed to demonstrate that they were more tolerant. However, if liberals were blocking conservatives for racist comments (like calling Obama a monkey), then it could just prove that conservatives tend to be more intolerable.
I want workplace protections for someone holding up an "Impeach Nixon" sign (or an "Impeach Obama" sign, for that matter), but you're right that racism along the lines of "White Lives Matter More" can reasonably be construed as job-related.
Racism in America could be treated like gambling in baseball. You don't do it. Or at least you don't get caught. Baseball players can argue that there are arguably worse things such as PEDs and electronic sign stealing, but... at this point, all of the players should know not to bet on baseball. It almost destroyed the sport. These other types of cheating are newer, people need time to adjust as we muddle through. The same goes for controversial topics like biological differences between the sexes, trans issues, and so on, but when it comes to racism, you should know better.
This goes for undsiguised racism. It does not apply to the UCLA professor put on leave because he read Martin Luther King's "Letter from a Birmingham Jail" (which includes a racial slur). Or to comics working out material in a club.
Bringing things back around again, what are we going to do with the racist receptionist? Do we follow her to future employers? Is she going to be unemployed indefinitely, in which case she's subsidized by taxpayers?