With this reasoning, why do we have more police around when a sporting event or concert gets over?  Or why is police presence more heavy at the court house when the verdict to a well publicized, emotional case is being handed down?  Or why did the police increase patrols around 16th street mall?  It is because if those in charge know where the location a crime will most likely occur then it only makes sense to deploy your resources in those locations.  Seems pretty reasonable and logical to me!
And how did they know these motorcyclists would have a higher proclivity towards crime?  Just by looking back almost 1 year ago and the actions from that time.
Would you want a police presence in your neighborhood if you knew in advance that Charles Manson and his followers were going to be walking down your street on a particular day?!?