Zero-tolerance at work: a student called a school security guard, who is black, "nigger". The guard said, quite properly, "Don't call me nigger." The guard was fired for this.

The guard did not use that word. Rather, he mentioned it, to oppose its use. I can't believe that the school officials fail to understand the difference — of course they understand it, just as you do. So why the absurd decision? I suspect they have bought into the idea of rigidity as virtue, which is the root of zero tolerance.

All rules, including valid rules, are liable to become harmful and unjust when applied rigidly.