> When I moved to the US in my teens, I was surprised at the sheer disregard of English grammar rules in common vernacular here, to the point that some folks (though not my English teachers) think their incorrect use is correct.

You noted a common feature of incorrect Americanisms, which is that many of its speakers proudly and ignorantly proclaim their usage is correct. Disregarding descriptive linguistics, all it takes is one second of logic to realize why an incorrect saying doesn't make sense at all, but even that is asking for too much without getting into an argument sometimes.