Power. To show that the police cannot protect anyone. The sniper was showing the cop, see, that 'could' have been you. The sniper gets to 'choose' who lives, who dies.
That is to say that most cops have AR-15s or similar rifles in their lockers and would use them to take out a sniper, if they can find him. From libkill, here
It keeps springing to me that this shooter might have issue with the Homeland Security, which the Senate began considering 10/1. I wonder what all recent assignments/budgets involve the .223.
I wonder also which exact Tarot card was left.