Since some of the people who were arrested had receipts showing that they were engaged in commerce with Sonic at the time of the arrest, then the cops did NOT see correctly. Maybe your supercops have ESP.
Although this is silly to continue this since I'm not defending the cops for arresting anyone, I'll state this again. I feel that it is highly unlikely, given that this was a known hang out for teens, that anyone in the age group of the crowd was not part of the crowd that had apparently been doing this for ten months.