It's not about politeness; it's about political power. In particular, the definition of "polite" is entirely asymmetric, with "marginalized groups" (basically, everyone but straight white males) given carte blanche to police others' speech and actions. In the case of Halloween costumes, you can see the asymmetry easily: a Native American man can dress as the Lone Ranger, but a white man can't dress as Tonto. (More precisely, a white man complaining that it's racist for a Native American to dress as the Lone Ranger would probably be ignored, and might even be called a racist himself.)
This has a rationalization, of course; it's OK because straight white males perpetuate "structural racism," imposing a "heteropatriarchy" on gays, women, and people of color. The practical result involves stripping power from straight white males and giving it to everyone else.
From an adaptive point of view, ripping off chunks of power and giving them to your supporters is a winning political formula. But pleasant nostrums about sticking up for the downtrodden are little more than a thin veneer over a good old-fashioned tribal throwdown.
This has a rationalization, of course; it's OK because straight white males perpetuate "structural racism," imposing a "heteropatriarchy" on gays, women, and people of color. The practical result involves stripping power from straight white males and giving it to everyone else.
From an adaptive point of view, ripping off chunks of power and giving them to your supporters is a winning political formula. But pleasant nostrums about sticking up for the downtrodden are little more than a thin veneer over a good old-fashioned tribal throwdown.