How's that spelt?

According to a study of children's writing by the Oxford University Press, young people are increasingly using American words.

I'm not that bothered about American words being used - it's pretty much inevitable given the influence of film, TV and music. What puzzles me is the seemingly recent spread of American spellings such as "city center" and "TV program". Where has it come from and why now? You can't even blame it on Australian soap operas as you can with uptalking.