From the article:
The New York Post originally reported that the first undercover officer to admit being at the ruthless pounding was not the only cop at the scene, as several members of the motorcycle group- called the Front Line Soldiers- are police officers themselves.....
So the New York Post did report it earlier, but doesn't allow posting on Free Republic. So it's really 'thank you to the Daily Mail for allowing articles to be posted on Free Republic, so Freepers can stay informed.'
NY Post is not a forbidden source - it just has to be excerpted.
"Front line" is a name that cops would tend to choose for a cop motorcycle group. It fits with the meme that police are on the "front lines" against crime.