Well, calling people stupid or insisting that they’re wrong when they aren’t isn’t likely to encourage learning or change. It’s the calling people stupid bit that Biggirl was Pitting.
*And, as I’ve said, very often the thing that is amiss is that people from outside their community keep trying to get them to admit that their native dialect is “wrong” and agree that SAE is the only “proper English”.
To run with the clothing analogy, let’s make non-SAE English dialects some sort of clothing not common at school or work in the US, but both traditional and popular within a certain minority population. It could be anything from a sari to a cowboy hat and boots.
There are many reasons why these clothes might not be acceptable in school or work situations. They may be a violation of the dress code. They may be a safety hazard. They may interfere with the individual’s ability to perform the required tasks, or interfere with someone else’s work. (A big cowboy hat could block the view of the student sitting behind you!) They may just not fit the image the company wants to project.
There’s nothing wrong with saying “I’m sorry Kusum, but that sari could get caught in the machinery. You’ll have to wear something else to work.” There’s nothing objectionable in “Johnny, our dress code doesn’t allow hats. You need to put that away, you can get it after class.” Some forms of clothing are inappropriate or unacceptable in some situations, and people need to know that and deal with it.
However, “Kusum, that’s not even proper clothing. You can’t come to the office draped in a big sheet. I don’t know what you people do at home, but around here we wear real clothes” is pretty insulting. So is “Johnny, only stupid boys wear cowboy hats. You’ve got to stop wearing that if you ever want to be successful. Have you ever seen a brain surgeon wearing a cowboy hat?”
If people are faced with the choice between selling out their heritage by agreeing that it’s wrong/bad/stupid or telling the rest of the world to go hang while they stick to the ways they’re familiar with, a lot of people are going to choose the latter. Even if it shuts them out of career opportunities or high society. They’re not stupid and they’re not oblivious. They have been faced with an unfair choice and taken the option that doesn’t require them to take sides against their own community.
That’s the way language choice is presented to a lot of non-SAE speakers. It’s the way it’s been portrayed by many posters in this thread – some grammatical constructions are “just wrong”, even if they are common and acceptable in some minority dialect, because there’s no “correct” way to speak English other than SAE.
This attitude is not merely one without any basis in linguistics, it’s prejudiced and unhelpful. It does little to encourage the use of SAE, and much to promote mistrust of SAE and SAE speakers.