Why is the workplace so special?
The workplace is special because people go there to earn a living, because without that paycheck they'll be homeless. Not that many people go to work just for the fun of it; most people go because they need money to survive, and would rather be sitting on the beach or hanging out with friends/family or just about anything else.
Because of this, society has decided that some rules are in order to keep being at work any more miserable than it already is, and to ensure people have a fair shot at being gainfully employed without being abused or forced out just because they're the wrong skin color or sex.
What people do on their own time is not subject to the same rules as when they're at work, working for an employer. If you want to be blatantly racist on your own time, you have that right thanks to the First Amendment. You might not make many friends that way, but if you want to be an ass, you can do that. On the job, however, you open the employer up to lawsuits if he doesn't take appropriate measures to stop your behavior on the job, so you'll most likely be fired if you say stupid racist shit there.
A free software github repo is not a workplace, and doesn't resemble one. It's some small project run by some volunteers. That's why this distinction is important.
However, while these morons are free to act as dumb as they want here, everyone else is free to criticize them.