We don't teach all the other things in textbooks either, like foreign language and punctuation! These are things the reader is expected to know or are things that they are learning in the class, just like the sighted readers. We list the symbols that are used in braille and then the braille reader sees the same symbols as the sighted reader. Some of all of the symbols may not be understood by the sighted readers either! Explaining symbols is the teacher's job. Making those symbols accessible to the braille reader is the transcriber's job.
Hope that helps.