What's the Cause of It?

Well it's raining in Ft. Worth--the first rain we've had in a very hot month, + thanks be to God--and it seemed the perfect weather for Audra to pop in "All Creatures Great and Small," an old BBC series she rented on Netflix. We looked at each other after one of the characters looked over at his brother and said something to the effect of, "In short, brother, you are at liberty. But I must needs remind you not to confuse this liberty...with license," and expressed our mutual frustration at the total inability of 99% of America's youth--all right, perhaps not that much, but still--to understand that sentence (or any that even remotely approaches it in terms of vocabulary) or to respond in kind. The result of this lack of common usage is that we must now use more words to ultimately describe what we mean, but what is the cause of it?

I find it odd (and sad) that even lower-class English citizens possess a larger vocabulary than most Americans...