The reason has to do with the underlying data type. In most systems there is no such thing as a "NULL" date/time variable. Its worth noting that even if the user deletes the value in this component, and then exist the form, in most cases it will still output whatever the default variable was.
Best solution I have found is to pick some sort of default value to represent NULL and then check for it once you exit the form. In many systems this is 1/1/1900 12:00 AM.
Hope this helps. If anyone has a more elegant solution for Workflow I would love to know.