It's plausible that Americans are worse at this than foreigners, but English spelling is a crime and everyone who makes mistakes ought be excused on account of how ridiculous the "rules" are.
I know how to spell most of the words I use and I resent the fact that my memory is polluted with this otherwise useless knowledge.
In many other languages if you know how to say a word, you automatically know how to spell it. We need a world language with grammar as simple as English and spelling as straightforward as German.