You underestimate people's ability to make language errors, including spelling ones. Every time I see somebody I suspect is a native English speaher using "it's" for "its", I grind my teeth. (Another instance is somebody using phrase like "as a programmer, the data bus should be written..." to mean "I, as a programmer, think that..."; this phrasing makes me simply furious.) With those errors they make reading my second language so much harder, and I can't even point their bad spelling or writing style out, because I'm seen as being nitpicking or something.