They have to confirm identity. That's why they make you sign a ticket and if you refuse, they take your thumbprint.
He was prolly telling you to get outta the way or to slow down. It just depends on how lucky you are. My friend is the luckiest guy in the world. Whether it's a lowered, dark tint all around Integra GS-R or his C32, dark tint all around, he'll be followed by a cop while going about 85-90 who then proceeds to pass him. I'd be pulled over in a second.