It's illegal because it endangers other people. People on bicycles, motorcycles or on foot are particularly vulnerable. People don't expect literal whisks to stick out from the wheels of a car.

Without even knowing which law regulating vehicle specifications is violated, I am positive that the German §1 (2) StVO is already violated:

Anyone participating in traffic must conduct themselves in such a way that no other person is harmed, endangered, or hindered or inconvenienced more than is unavoidable under the circumstances.