Because it's utter bullshit.  Ponzis are illegal for a number of reasons, deception being only one of them.  For one matter, a Ponzi is, as a matter of law, a business operating while insolvent.  For another, fraudulent schemes depending on the behavior of others have been considered violations of the Lottery Act, as they have all the criteria (chance consideration and reward).  The policy considerations are that Ponzis invariably collapse eventually, and often create havoc when they do, with bankruptcies that go on for decades costing enormous amounts of court time and money.