My guess would be that most of the protesters aren't German. (I was very surprised at the graffiti in the St. Pauli video, although thinking about it that area of Hamburg has always been kind of craphole.) I'm betting that many are the same ones that go from summit to summit protesting, and leaving behind a giant mess.
The thing is most Germans very much are clean and orderly (In Ordung, is the national philosophy), and have little patience for litter or other lack of cleanliness. Soldiers that lived off post would get in trouble with their German Landlords if they weren't doing their part to keep the outside of the building clean and swept. (And yes, they got Counseling Statements from Squad Leaders or the Platoon Sergeant if it got bad enough...)
So yes, cars on fire, broken bottles, and other trash from these protesters, I'm shocked the Polizei haven't gone all jackboot on these 'tards.