As someone from the Netherlands I can say there's a non-zero chance that a motorist has a gun. It's smaller than the US, but it's not zero. Still our cops carry guns. In the US the chances are way, way higher. I can understand that cops are armed there.

Because the cop showing up doesn't know before hand whether or not the motorist have a gun. There's a lot of other things they don't know. The person could be a drug/human trafficker, or have a warrant out for him. In which case, they are armed.