Your question implies that ALL white people are dogmatic about grammar, which is like saying that ALL black people speak ebonics. You are correct that grammar is not linked to intelligence but that’s the perception, right or wrong. And it doesn’t matter the race of the language-mangler. Speaking non-standard English can give the impression that the speaker is ill-educated. As an employer, I would not hire someone who cannot or will not speak standard English. I look for people who can communicate well and who will represent my company in a professional and positive way.