[Cerek the Barbaric]

They probably could have, but after searching the car and finding nothing, it was probably their way of "saving face". Yeah, it's petty and they could have overlooked it, but they're also within their rights to issue the ticket.

Several years ago, a college buddy of mine and I decided to take a road trip to Raleigh, NC. He had a brand new Mustang SVO and - on the way back - was driving around 85-90mph. Suddenly, his radar detector lit up like a Christmas tree and we saw a cop cut across the median about a half-mile in front of us (which wasn't very far considering the speed we were doing).

My buddy was SURE they had caught HIM on radar, so he got hard on the brakes and took an exit that happened to come up before we caught the cop. We pulled into the convenience store and spent about 30 minutes waiting for the trooper to give up and go on down the road. He didn't.

He was still waiting for us when we came back down. The only problem was, my friend had hit the brakes before he could get an "official reading" on his radar. He was sitting on the side of the road and pulled out after we passed. He followed us a little way, pulled out as if he were going to pass, then dropped back behind us and "blue-lighted" us. Seems my friends tag had expired, so the trooper was able to give us a ticket for that.

But his "real" message was to let us know the he COULD have had us for a much worse violatio.

Just like Arledrians example, I'm guessing they had a different reason or suspicion for pulling your friend over, and the taillight was just a convenient "back-up" in case their suspicions were incorrect.