It is standard boilerplate. It also gives them cause to fire you if they can determine that you were a source of an internet post that they found objectionable, whether or not it was true. Plus, I suppose it lets them say they fired you because you were a disgruntled employee.
This might have been a concern for Danny's proposed Porcine Aviation "Professional Pilots ONLY Forum." Managers with the authority to fire employees might be airline pilots able to join closed forums.
In the United States, the courts can and do force internet service providers, such as AOL or Yahoo!, to disclose the real identity of "anonymous" posters if libel (or criminal intent) is alleged.
Edit to add: Glueball has a point.