The conspiracy theories about power signalling in the other comments are all well and good, but I have a simpler theory: U.S.-ians don’t know their own language, plain and simple. There’s a reason that American English is referred to as “English, Simplified”. The U.S. is a culture of barbarism, and language is one of the more conspicuous areas where it shows. I’d suggest that those who still know grammar and orthography are less victims of a power imbalance than just better educated. Or maybe their tastes aren’t as badly ruined yet.