Politeness (under which requests for the "right" "tone" seems to fall: it's not specifically a soft tone or a pleading tone or whatever that's requested, but something more abstract, especially in written discourse) is a product of cultural conventions that grow out of historical and political dynamics. It's a product of society: it reflects the order established in that society. Discourse that upsets the dynamics of society fails to comform to that order. (Sometimes it's on a micro scale, sometimes it's on a macro scale.) There is no way that anti-oppression talk can come off as polite, because it upsets the established order of a society based on oppression. And the rudeness of the oppressors toward the oppressed is invisible as such because it doesn't upset that order (as well as because the people at the top have the material means of getting away with it).