Or it is an act of civil service.

Driving with an obscured license plate is illegal and could get someone in real trouble. Obviously nobody would want to intentionally break that law. By removing such an unintentional obstruction they are doing the owner of the vehicle a favor - provided it doesn't damage the car itself, of course.

Similarly, if you see someone throwing trash out of their car, picking it up and depositing it in a trash can does not count as theft. Removing a third-party sticker from a traffic sign does not count as damaging it.