You have been breaking the law before they contact you. They are just sometimes nice like that. In contrast to the

If you want to know the specific part of the traffic law: StVO § 14, 2 (safety when entering and exiting the vehicle) [http://dejure.org/gesetze/StVO/14.html]:

"Kraftfahrzeuge sind auch gegen unbefugte Benutzung zu sichern."

(Vehicles are to be protected against unauthorized use.)

Also, your insurance might not like this ;).

As I said before, the german law is much more build around responsibilities. This is one example.