Conspiracy only applies if a crime has been committed or attempted. At the time of the raid,the cops had no evidence that a crime,an attempt at a crime,or a discussion of a crime had taken place.

As far as being Mafia members,remember that no court of law proceeding had ever proven that the Mafia even existed. That came much later in our history.The cops couldn't use "Mafia membership" as a cause for arrest

Finally,merely meeting is not a crime unless you are in violation of parole by associating with known Felons. If the heads of the New York Families all went to dinner together at Rao's,(assuming none of them tried to beat the check ),they would not have done anything illegal.