Judging from the pic of the plain white Crown Vic the officer was in, could the assailants have thought the officer was a civilian or a wannabe in a cop surplus whackermobile and wanted to dust him up without fear of retribution? I doubt they would have fired if they would have know it was a real cop, although standing on a street corner with rifles does show some indifference to the law.
In response to Amanda's post below, I saw the pic of the plain white Vic in the article and didn't go to the slideshow, so I just assumed he was driving an unmarked.