If you can’t write it means you can’t think, it means an important part of your mind is not working very well. It’s separate from class issues, because millions of books are basically free now, and if you can’t access them there is the web or old newspapers and magazines lying around. Plenty of people from poor backgrounds write perfectly well.
It’s not a literary matter either: there be folk who can do some real fancy stuff with words and grammar. But we don’t have to be able to do that to communicate something clearly, and most times that is exactly what we ought not to do if we are after communication rather than praise.
Finally it has nothing to do with being English or American or black or white, nor even whether you are a native speaker or not. Those Britons who howl at Americans degrading their language are idiots who need to read Faulkner. Those Americans who are impressed by a British accent are idiots who need to grasp that Ricky Gervais is not Bertrand Russell.
Much as it drives some people mad, the fact that you cannot construct a sentence that carries your meaning tells the reader something important about you, and you can rage all you want about snobbery and elitism without changing that fact that writing is thinking + keyboard and, face it, there is nothing wrong with your keyboard.
Grammar Nazis are jerk offs, everyone knows that, and typos don’t matter until they start to rival non typos when you are sober, and if you are not sober then you may need to get a grip for reasons that have nothing to do with writing per se.
But all of that said, the fact that nationalists, conspiracy nuts and overly defensively religious folk tend to write badly is as unavoidable as it is predictable. There’s a moment in Fight Club where Edward Norton’s character listens for a minute to a man in a dry cleaners failing to answer a question: “I'm not exposed to bespeak any such information to you nor would I, even if I had said information at this juncture,... be able…” And Norton’s face drops in weary recognition at what is going on and says. Resignedly: “You're a moron.” Then leaves the shop as the guys says “I'm gonna have to ask you to leave.”
That’s how it is on a forum or a comment thread or a chat client.