The designers of CORS kept the previous security for certain types of POST requests because they are basically equivalent to how forms work, which operate at a much worse security standard and are a source of a great number of CSRF problems. The simplest way to remember it is that most things which break the legacy behavior of POST triggers the preflight. Speaking of form problems last I checked HN has a login CSRF.
It will trigger preflight if POST and not a form content type. (E.g. a POST application/json)