[strken]

> If native speakers say something, then it's grammatically correct.

In their dialect, sure. In any given dialect, who knows?

Any speaker of a dialect that isn't West Coast American has likely watched actors who live in Los Angeles try, and fail, to speak their dialect.

[umanwizard]

Of course. Just like what I’m writing now isn’t grammatically correct Spanish or Chinese.

[strken]

Well, no, but I can't understand 99% of Spanish and Chinese using my knowledge of my native English dialect, so I'm not going to lecture Spanish or Chinese speakers on correct grammar.

My point was that differences between dialects drive a lot of arguments about grammar. If I'm annoyed about the phrase "could care less" then that originates in my native dialect not allowing it. Not being part of the prestige grammar is secondary, as is whether the phrase makes any logical sense.