Language, like food, does not stand still. It evolves and what was once unacceptable becomes acceptable with common usage. If it did not we'd all sound like characters from Beowulf, or the Canterbury Tales, or whatever time period you feel freezing the language is appropriate. To quote Professor Brians, "When you reach the point that nobody seems to agree with your standard of usage any more, you may have simply been left behind." It may not be "nobody" on that "au jus" island yet, but the population is getting thin.
But the US is not the only island where 'au jus' crops up (for example, there are entire countries where they speak French correctly), it isn't even your own language you're mucking about with.
There's a substantial difference between trying to freeze a language at a point in time (a few countries have made that effort), and avoiding the perpetration of wilful mistakes. I'm a copyeditor, so I hear the argument you've made all the time; it's as though people believe that no nation exists other than their own. You might argue that with the many idiotic terms that are are tolerated in the culinary world, this is nothing, but if you visit France, and ask for your whatever 'with au jus', and your waiter rolls his or her eyes, are you then going to complain the French are rude and arrogant?
Language shifts, it's natural. But if you know that something is incorrect, it just makes no sense to defend the mistake, you suck it up, and avoid it in the future. I know what I'm talking about, because for a lot of my life, I've been learning one new language or another. Some of my mistakes (e.g. 'snot papir' to mean tissues) have been adopted by friends in a joking way, because they're funny and communicate clearly; most are dinner stories (my confusing the Danish for 'bra' and 'necessity').
Most countries do odd things to culinary terms from other languages, I've heard some beauts in Italy (I once spent most of a day trying to figure out 'peenat batr') and Denmark (Danes use 'grape' to mean 'grapefruit', which makes for some confusion when they travel outside DK); this isn't unique to the US. But regardless of where you are, or whose language you are attempting to use/incorporate into your own, it just makes no sense to run with what you know to be incorrect, then say 'Eeverybody is doing it' (unless you're 14 or so, then you get a pass
). That's not an argument: It's a really weak excuse for laziness.