First off we need to be clear: false complaints of harassment, especially of sexual harassment, are rare. While an employee may be sharing their version of events as they remember them, it is unlikely that their story is completely fiction.
However, that does not mean that employers need to take employees automatically at their word. There are situations that arise where an employee will make false allegations in bad faith, usually to punish an individual who they feel has wronged them. Naturally since it is so important to take complaints seriously, complainants automatically receive the benefit of the doubt. However, employers should always conduct whatever investigation is necessary before taking disciplinary action against an accused.
If an investigation proves concretely that an allegation was made falsely in bad faith, employers can penalize complainants for such conduct.