I agree. Anyone can speak a language badly.
The job of Duo is to teach what would be taught in courses in their schools that teach Czech to children.
I teach English writing. For English, Duo's job is to teach what is taught in English classes.
As a teacher (in a university), I know that many Americans can't write well, and many speak with bad grammar. That is why, in a university, students are required to take classes in composition (English).