When an officer approaches a car for a minor violation, they almost always touch the taillight.?

There not touching the taillight and they aren't doing it to leave their fingerprints. Officers are checking to make sure your trunk is secured. There have been many instances where gang members hid in trunks, and then had the driver commit some minor violation in front of an officer so they get pulled over. Then the gang member would pop out of the trunk and ambush the officer. This was very popular in CA, and now police academies train officers to slightly push down on the trunk to make sure it's secured.