Richard, I see the following very clearly mentioned in the doc:

"If you want a given stdin to be used, make sure to set the instance’s
use_rawinput attribute to False, otherwise stdin will be ignored."

Even though this seems like unnecessary, at least it is documented. If
you want to push for automatically setting use_rawinput when 'stdin' is
not None, you will need to submit a patch and convince some core
developer to agree with you.