1/ When it comes to pronoun shit, and 'what do you call people of X ethnicity when physical description is needed' shit... well why doesn't politeness flow both ways?

Southern folks are real big on politeness, but the way it works... it's two way.

2/ So if someone commits a minor breach of etiquette, it is polite to either ignore it, or gently remind the person in a non-embarrassing way, that it was impolite.

Call out culture is basically doing this meme. A Southerner would consider it the height of impropriety to do this

3/ Among SJWs, politeness is thus a one-way street. If someone breaches SJW etiquette in some fashion, the response isn't to gently remind the person/ignore it/whatever.

It's to screech it as loudly as possible and ruin the offender's life - or at least make him a mob target.