Because it's annoying to see some one get attacked for a mistake on your part. You misunderstood the meaning of a sentence, not a big deal. But trying to argue over faulty grammar, when it isn't even a grammar error, is just very rude to be honest.
Also, you very obviously stated that with a very noticeable sense of superiority. You proclaimed your dialect of the language as the "real" version, while ending the sentence on "whatever", which clearly implies that every other dialect is a "lesser" form of English.
I'm also not sure where you get your information from about what's being taught in schools. Every school in Europe teaches British English, even Canadian schools teach it. If some school is for whatever reason teaching American English by choice, then it's by no means a standard for the rest of the country.