I grew up around a lot of people who are relatively obsessed with European-derived etiquette and have been raised thinking it's the "proper" or "correct" way to do things. Very bourgeois attitudes, if you will. I used to be like this as well in some regards; in terms of speaking I had it drilled in my head to
enunciate, and that language is far less fluid/dynamic than it actually is. A lot of folks seem to not be able to function outside of a clearly written list of rules (you can see this also come out in them when they act like social dynamics don't exist and just resort to "is there a law against this thing I did?" to defend their actions). I also know some rednecks who mock any kind of slang that they aren't used to because they see it on the Internet and associate it with attention-seeking youth who "don't know what the real world is like" or some shit like that.
Fortunately as I grew older and I started spending more time online and hanging around more people with varied dialects, and was introduced to their respective cultures, I dropped that shit. But a lot of others didn't, and still lord over other people because they see slang or dialects as "uneducated", "uncivilized", "classless", "immature", "thoughtless", or whatever other way you can word that kind of judgment.
Considering how slang directly reflects one's environment, demographic, and culture, any complaint about innocuous slang always has an air of prejudice to me. Like this person has pent up disdain toward a group of people, and this is a way they can indulge in it with less chance of blowback.
As I've learned more about dogwhistles this started becoming apparent as well. In my area I think it's both expressing a pre-existing disdain for a group of people, as well as expressing disdain for the slang because it's different/perceived as "not proper" (especially since there is little diversity where I grew up which encourages homogeny) which ends up being attributed to whatever group speaks it over time due to it being a product of the influences you described.
And I think that latter part is an aspect of racism that a lot of privileged folks don't understand is racism because (again going back to the "clearly written list of rules" thing) it doesn't fit the dictionary definition of racism being personal prejudice. But looking down your nose at others' use of language because it's different than yours - assuming everyone should adopt
your strict standards regardless of someone's culture or the internal consistency of their slang/dialect, refusing to acknowledge the dynamics of culture and language and all of that, and assuming someone is somehow lesser because of their different use of language - is thoughtless, needlessly dismissive, self-absorbed/self-righteous, and it contributes to negative stereotypes and carelessly marginalizes other peoples which in turn perpetuates prejudices including racism.
It's just different ways of expressing one's self ffs
So when I see someone who is bothered by slang, part of me assumes that they're either too stuck up to accept that their way isn't the only way, are extremely boring/vanilla people, or are simply prejudiced.
