Because Chinese is your second language.
eta: A rhetoric class in your native language might affect how you speak in certain situations, but it wouldn't affect how you spoke at home or with your friends. Native, everyday language is very unsusceptible to external mandate. It is, however, very susceptible to peer groups and culture. We weren't originally talking about rhetoric classes, though, we were talking about English teachers.
This is a very touchy subject for me because of some of the crazy political nonsense that's going on recently. Arizona was trying to pass a regulation that teachers who spoke with "foreign accents" couldn't teach elementary school because they would teach the students nonstandard English. One of my linguistics professors posted on facebook a letter written by a coalition of linguists repudiating, point by point, the ridiculous assumptions of that policy. I was trying to find that letter but I haven't been able to again.
Annie, I for one don't believe there really is a "standard English". I have no problem with people with accents teaching English because by and large there is so much shared media it creates a sort of standard we can reference when we speak to each other, rather than pockets of America where there are more and more distinct accents and vocabulary.
But you are talking to somebody whose accent and speech patterns are very fluid. When I talk to people I quickly pick up on mannerisms and methods of speaking. It makes people more comfortable in many instances. I have a completely different voice (for lack of a better term) for each one of my siblings. I've always been that way as long as I can remember. I get that I might be a bit of an anomaly to a significant extent, but I am also convinced that if one took a class where they were taught rhetorical principles and how to speak in public, you would find that adjusting your speech patterns is a skill just like any other. If instead of foreign languages we insisted on foreign accents in school, people would be quite capable of completely adjusting their accent and sticking with the new accent even as their primary voice.
When I took a rhetorical class in high school, my teacher told me that I had a weakness with using inappropriately advanced words when speaking to an audience. To a great extent I was proud of my vocabulary, and frequently used five dollar words when speaking. I had to spend the entire semester working on it, it was crazy hard to not use those words but also not sound like I was speaking like my audience was full of dumb people. Since then, and this is still true today, I'm much better at adjusting my word set depending on who I speak to.
Again, that's all on top of my accent, and manner of speaking.
