I'd argue that "I don't mean to X ..." has actually fallen victim to the euphemism treadmill ( https://en.wikipedia.org/wiki/Euphemism#Euphemism_treadmill ). It can still be used earnestly for its original meaning, but that usage has been overtaken by the usage where someone essentially says "I'm not racist but - " and then proceeds to make a racist remark. This leads to the situation that your parent comment points out.
Language, of course, varies, and we should keep in mind that some uses tend to crowd out others - but I definitely think that this is something like the euphemism treadmill at work.
I think it's also a victim of political correctness, which is closely related to heavy use of euphemisms. In many places these days, it's pretty difficult to make certain kinds of observations or relate certain kinds of anecdotes without somebody taking offense. "I once had an African-American roommate in college and he was..." "You racist!""I met this crazy girl the other day and..." "You sexist!" No wonder people feel like they need to qualify their statements.
Why do you explicitly point out race or gender? You don’t have to, you know. It’s kinda irrelevant most of the time. (And if you think it isn’t when you are telling some harmless anecdote it’s possible you might be racist or sexist.)
I do mention that all the time, because it's simply part of the story, and that is how I tell stories, with all kinds of (potentially meaningless) details. If people think I am sexist or racist because I mentioned that a "black girl pulled out a knife", then so be it.
Why do I care that he was your high school friend?
I don't mean to be rude, just saying - people use such descriptions intuitively (because it makes the story flow better), and it's not always because they are prjudiced towards some group of people.
That he is a friend from high school tells you about his relationship with me. It answers the question, "Why do you, carbocation, care what this person thinks?"
Note that I didn't tell you about his skin color, the color of his hair, or anything else that wasn't actually relevant. Prejudice is a tough nut to crack and is not the same thing as bigotry.
In that case, you could just tell me he was your friend, not high-school friend. Or better yet - "Person I care about ...". If being too specific was bad.
Anyway - I don't see what this has to do with prejudice or bigotry. People add details to stories, because that makes stories better (at least in the opinion of the people that tell these stories).
I live in Poland, there are very few people other than white here (like, I've met black people maybe 10 times in my life), and when someone tells a story, nobody mentions race (because it's assumed to be white), but people tell "So this big bald guy says ...", or "And that gray old lady did ...". That's just how people tell stories. Is this prejudice, too?
And if I met black person and something interesting happened, I'd tell in my story, that this person was black, because I'd certainly remember that as a distinguishing characteristic, exactly like being big bald guy, or being grey old lady. Or I could skip "gray", because "gray" is the default visualization for old ladies.
That's a valid question, but it's just as applicable to any incidental details. I imagine other details like when or where you met the person are equally irrelevant, and yet including them is completely reasonable.
You wouldn't say, "a two armed person pulled a knife on me!" because two arms is the norm, outside of, perhaps, prosthetic clinics. So, I assume that "black" and "girl" are perceived as exceptional enough by the speaker to warrant mention.
Personally, I'm more concerned with the tone and context. My extended family insists they aren't racist, despite spitting out the word 'black' as if it were stinging their tongue. Also, they like to casually mentioning how much they dislike the "beady yellow eyes" of Indians (maybe), and integrate the n-word regularly into their humor.
People don't usually acknowledge it themselves, but that's how oral narration works - it has to keep the listener's interest and relate to them, be it either an anecdote or something that happened to one yesterday. Compare someone asking for accept ("permission" for telling the story): 'Hey, know what tall black guy living next to me? [audience reaction] So, yesterday we went to...' vs 'At a certain time, I went with this person to...' (you can add M. and N., if you want - like in Chekhov novels) The other version lacks any way to relate to listener(s) and would feel pretty dry.
Personally, I just want to be able to accurately picture what happened. If I later find out that some character in a story has a different appearance from what I imagined, there's a weird adjustment.
Somewhat related to this subject of the link between concepts and words/phrases, the utility of clichés, etc., I strongly recommend Douglas Hofstadter’s lecture, “Analogy as the Core of Cognition”, http://www.youtube.com/watch?v=n8m7lFQ3njk
Language, of course, varies, and we should keep in mind that some uses tend to crowd out others - but I definitely think that this is something like the euphemism treadmill at work.