You consider it "mature" to address the entire population of nation with an accusation that they don't speak their language correctly? Either you've got a lot to learn, or you're incapable of learning. Many native speakers, such a me, say "could've," a contraction of could have. That you can't tell the difference by ear is hardly evidence of gross grammatical crime on the part of every last one of more than 300,000,000 Americans. I have, many, many times, seen native speakers who happen to be English, or Canadian, or Australian who do not properly distinguish between they're, their and there, or yore, you're and your. Why don't you just rant against the more than half a billion native speakers of English in the world, and have done with it?
More than that, communicative competence is all that really matters in a spoken language--does your interlocutor know what you meant? It may be more important to get it right when writing, but a competent native speaker will still usually know what was meant. Do all the speakers of your native language always speak it properly? Are there no dialects or regionalisms? Do they always write the language letter perfect? I doubt it.
Frankly, i find your remark to be snotty and xenophobic--there's a lot of people who seem to get their jollies bashing the Americans.
Yeah . . . tell me about maturity.