It's pretty hard to initially digest, but when you think about it, right and wrong with language is almost entirely just convention. The difference between what we consider 'bad' English and dialects of English is dubious at best.
Natural languages are not designed. Pun intended. Afaik languages are more or less cannonised after people have already started speaking the language. If there is a large enough group talking in a certain way who's to say it's wrong. Every word has at one time been made up. If for example enough people decide they don't care about "their" or "they're" then "their" it is (or "there")
I did not major in linguistics but I had some very engaging linguistics professors at Rice. Linguists consider themselves to be descriptive and typical language teachers in grade school to be prescriptive. In other words, linguists take pride in describing what native speakers actually do in practice rather than what they are instructed to do. So if they catch a native speaker doing something the "wrong" way they'll classify it as an error only if native speakers will agree that the example in question was a mistake because it doesn't make sense in the dialect. If native speakers agree that whatever the speaker did actually does make sense to them, e.g. they're fine with the usage of "ain't" or split infinitives or whatever in everyday speech, then linguists will consider that to be a genuine example of the dialect. But linguists are quick to point out that what is correct changes all the time because words and phrases used by native speakers naturally erode and morph and new ones are adopted, and trying to nail down what is correct is futile.
Basically I look at it this way: in reality, the coordination of 'correct' language is a practical consideration, and it is an error to consider it a matter of prestige, as if it's decidedly unintelligent to deviate from the established norms.
