Re: df is called even if dired-free-space-program is nil ...

[Richard Stallman]

    Please note that putting the code which produces the free space into
    insert-directory means that any file handlers which implement this
    primitive need to mimic this aspect as well.  This might be a Good
    Thing, but it's a change that should be documented somewhere, I think.

I think this goes without saying, because any file operation handler's
job is to mimic the behavior of the Emacs primitive it is handling,
and it will be plain enough what that behavior is.