We're not talking about some kind of loitering with intent crap; these people had a clear, premeditated intention to break the law. If you announce publicly that you're going to commit a crime at a given time and place, don't be surprised if the police turn up to that place at that time and arrest you.
Did they say things would get unruly? Did they say minors would be drinking? Did they say the neighbors would complain because the music is too loud? Did they say they would be breaking fire codes? What crime did they publicly announce? The police are making a lot of assumptions (right or wrong) before the fact.
