So, I have a PhD in linguistics.
There's two ways to approach language, particular in terms of grammar: prescriptivist (this is what language
should be) and descriptivist (this is what language
is). A prescriptivist says that "we be hanging out" is bad English; it's not in the accepted standard dialect/register. A descriptivist says that it's perfectly good English, it just belongs to a particular dialect/register (African American Vernacular English in this case). "We be hanging out" is perfectly grammatical in AAVE; it's not grammatical in Standard American English. That's because they aren't the same dialects.
Linguists pretty universally approach language in a descriptivist manner, because we're concerned with language as an object: How does language work? What is the mental object that is language? For the sociolinguists, what is the
social object that is language? Language is immensely diverse, and amazing, and we want to know all about. When you study language for long enough, you discover that there is literally nothing that makes one language or dialect better than another. "But German sounds so harsh!" -- that's an aesthetic judgment, and therefore it has all the weight of any other aesthetic judgment (i.e. basically zero). "But such-and-such people have eleventy billion words for X!" -- well, you're probably confusing "word" with "sentence", which is very easy in some languages. There's no one language that has special expressive power, though, and there's hundreds, if not thousands, of papers out there showing just that.
Most of the rest of society approaches language from a prescriptivist position. This is where power gets involved, and as Malkynn points out, the "preferred" dialects are (in the US at least) inevitably white and upper-class, and if you go deep enough, typically coastal (e.g. white upper class California English is becoming the standard TV dialect, for example). That's because these are the dialects spoken by the people in power; everyone wants to speak like the people in power, after all.
Insisting that the only "good" dialects are the dialects of people in power necessarily makes a value judgment on the dialects of people NOT in power. Those dialects, the dialects spoken by the poor/brown/gay/whatever people, those are the BAD dialects. That's where you get charges of racism, classim, and all the other -isms coming in. You're literally telling people that their language/dialect is worse, and you're making that judgment is based on power structures. People don't think of it that way, and it's very rarely expressed overtly that way, but that's what's happening.
There's plenty of value in learning the dialect of power. But acting like the dialect of power is the One True Dialect? Yeah, you're punching down in the power hierarchy.
@BDWW It's not racist to teach white people not to use ain't. It is, however, classist, and in the way you described it, urbanist, to tell them that "ain't" is inferior. "Ain't" is perfectly grammatical
in the dialects that have it. You can tell them it's not part of Standard American English and shouldn't be used in that context, but
Also, a gentle reminder that some of the people on these boards are the exact people you just made fun of. I am a white, lower-class person from Appalachia. I find your post deeply unfunny. You used two shitty words ("hillbillies" and "rednecks", which I've never heard anyone outside those communities use in a non-shitty way) and a parody of rural white English as a way to -- what? Make a point by making fun of folks?