The officer's training would instruct him to be wary of a gathering crowd. While you wouldn't expect a Cambridge gathering to be in full "kill the pigs" mindset, he's already got one guy who seems to be a little off the plumb of acceptable social behavior, maybe he's got like-minded friends.
To be consistent then, because cops routinely get shot at, the Second Amendment should only apply to peaceful hunters, with a temporary government permit of course.