> that's the pretty standard behavior on unix.

Not even slightly:

  $ mkdir 1 2
  $ touch 1/foo 1/bar 2/baz 2/quux
  $ mv -T 2 1
  mv: overwrite '1'? y
  mv: cannot move '2' to '1': Directory not empty
  $ mv --force -T 2 1
  mv: cannot move '2' to '1': Directory not empty