These are not the only differences between the written forms of American and British English. Several common words are spelled differently in Great Britain and its former colonies, as well. For example, words like honor, color, and favor are usually spelled honour, colour, and favour, and in words like criticize," the "z" is replaced with "s."
Or consider the word "spelt," an older participial form of the word "spell." You almost never encounter "spelt" in the United States, but it is actually fairly common in England. On the other hand, the older participial form "gotten" is almost unheard of in England, but in most sentences it is the preferred form in the U.S.
At least most Americans should be aware that the British spell "center" and "theater" as "centre" and "theatre," since so many businesses in the U.S. try to make themselves seem more sophisticated by pretentiously using such Britishisms in their names.
You would think that Americans would be familiar enough with British texts to realize that such differences do exist, and that when our friends in England, Australia, New Zealand--or in India, for that matter--follow the rules of usage that are appropriate to their own countries, they are not committing errors.
People trained according to British usage seldom try to correct Americans for following American rules of usage. If the differences between the two systems even come up at all, they are likely to refer to such differences in terms of a question--something along the lines of, "Is that how it is done in America?" But usually the issue doesn't even arise, because they read books, papers, and magazines printed in the United States, and they are well aware of the differences.
But for some reason, many Americans are oblivious to the fact that such differences exist, and when they stumble across something written according to the British system of usage, their immediate reaction is that the writer has screwed up and needs to be corrected.
And that's not even the worst of it. There is an unfortunate tendency among Americans to adopt a hectoring tone when they take it upon themselves to correct other people's grammar or usage. I have heard that tone blamed on American chauvinism, but actually I don't think chauvinism is what causes it.
Oh, sure, sometimes provincialism and even chauvinism are involved in the failure to recognize that the American way of doing something is not necessarily the only way, the right way, or the best way. But I think the real cause of that prissy, hectoring tone is that all through their years in school, American students are taught English by people who believe that writing is rule-driven, and that their job is to make sure that their students don't get away with breaking any rules.
Unfortunately, English grammar and usage are very badly taught in the U.S., even at the college level, so it is often the case that a person charged with teaching the subject is not really all that knowledgeable about it himself. Typically, English teachers, as well as teachers in many other subjects, have learned a bunch of "rules" (some of which are actually not even real rules**), and they rigidly enforce those rules with their students.
And some of these "enforcers" are downright fanatical! One will declare, "Death to all fragments!" Another will brag that no split infinitive or comma splice has ever made it past his watchful eye. I know one retired English professor who publicly belittles anyone he catches saying, "The reason . . . is because . . ."--a locution so ubiquitous that it can be considered an error only in formal writing contexts, but certainly not in colloquial speech!
When their entire educational experience has shown American students that grammar and usage errors are to be pounced on and mercilessly destroyed, then it is no wonder that they assume the persona of the draconian enforcer of rules whenever they think they have caught a grammar scofflaw thumbing his nose at decency and order as represented in the well-wrought sentence.
Under such circumstances they feel it is their duty to make a citizen's arrest, using as much force as necessary to subdue the miscreant.
I would suggest to my fellow Americans that if we feel the need to correct someone else's writing, we should first be quite certain that we know what we are talking about; and second, we should make our suggestions politely and respectfully. Even if it is the case that we are addressing a genuine error rather than merely a difference between American and British usage, there is no excuse for bullying or rudeness.
You didn't like it when your English teacher took that tone with you, so there is no reason to suppose that anyone else will like it either.