Why do Americans often fail to realise that the spellings they use are not the only correct ones?

It isn't just Americans. It is also true in most countries of the English speaking world. The US and UK do not have language academies to regulate the language as in France, Spain or many other countries. Children in the US and UK are taught a different standard in local schools. Teachers often insist on children following what they teach, even though it changes, Pluto is the same old Pluto, but please don't call it a planet.

If a student in the US or UK is lucky enough to attend a school that requires writing and uses an alternate standard, it will probably be marked as incorrect. Microsoft provides alternate spell checkers.

I am a citizen of the US, but do business and enjoy the culture in the UK. I often use alternate spellings because I am describing different things. "The word is not the thing." I might write "Broadway theater" and "West End theatre"; Department of Defense" and "Ministry of Defence" in the same sentence. My secretary only asked me about it once. I think she is smarter than I am.

Australians understand this. They spell the name of a major political party there in the US way.

http://www.alp.org.au/

Watch the upcoming UK election. Punters and bookmakers are wary. I don't think a change in spelling will save the party in the UK with a similar name as it declines into irrelevance. . Auto correct tried to make "irrelevance" into "irreverence," but I insisted.

I'm reconsidering. Perhaps Auto correct has a point,..

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An American trying to tell a Englishman how to spell is a bit like a Mexican trying to tell a person from Spain how to spell in Spanish. The Americans just bastardised the English spelling.