accident is one possible justification, and there's also a matter of
meaning.  if i say `copy this file to that file' i expect to see a
strict copy of the file as a result.  it's fairly clear cut.  on most
systems that provide directory `copying', if i say cp dir1 target (or
cp -r) and dir1 exists in the target, i usually obtain a merge (it's
perhaps one of the reasons that options bristle as finer control over
the operation is provided).  arguably, cp should instead remove files in the
existing destination directory in order to produce a strict copy.