I can't think of any dream I've ever had where there was any text to even try and read. Common knowledge about "how dreams work" doesn't seem like an exact science - like you said, you can read in dreams.
In the movie Waking Life there is a claim that you can tell if you are dreaming by trying a light switch, and that light switches don't work in dreams. I had a lucid dream where I tried it and the light switch did work. So I don't think there are any rules.