But tab characters are rendered by the terminal into spaces. During stdout processing, when the term encounters a \t (0x09), it inserts (into the term buffer that is displayed to the user) as many spaces (0x20) as needed to move the cursor to the nearest tab-stop (setterm --tabs will display them for you). Why do we need to duplicate this inside Python?

There are no copy&paste issues either, try it yourself: when you copy and paste tab-indented text from the terminal, your text will contain spaces instead of tabs (at whatever width you have your terminal tab stops configured for).