Well, their job was to teach Standard English.
Our society is organized in a way that relies heavily on having standard versions of languages. There's nothing wrong with that, but there are
problems when "standard" is confused with "correct."
In the first place, it's inaccurate to say that the standard version is correct; it's merely privileged. From a cognitive and neurological standpoint, all dialects and languages are equal because they're all rooted in the same ability to create, learn, and use language.
Secondly, by designating the multitude of nonstandard versions as "incorrect," you're disparaging and disenfranchising the people who use them. Since the standard version is determined by the powerful, calling all other versions "incorrect" becomes a justification for that power.