Yes there are loads of nice appreciative collectors and they are doing us a service, it's just sad when the predators come and ruin it.
Those cops are also regularly working that area (they in fact were already in the park when I called, and I had seen them before doing their rounds), and if that bottle collector violated their order to vacate, it would have had actual consequences. And if he pulls the same shit again, the cops will have a "you again moment", and probably mention that to the next person who makes a complaint and at the very least issue a much longer order to vacate. And since they didn't mention anything to me, I am assuming it was the first time the cops were called on that guy, and we Germans have a tendency to call the cops quite often ;)
It also saved me from a lot of hassle myself, of course, not having to go to the police to give a signed statement, maybe show up in court, etc.
One collector gave me a sticky, wet butt, because he snuck up behind us in a park, "stole" my half empty beer bottle I placed next to me, and then proceeded to empty it out right there. It was one of the very few times I had trouble controlling myself not to hit somebody... I managed to control myself in the end enough to avoid violence, but was still angry enough to call the cops on the guy. Cops told him to not show up in the park or surrounding parks for the rest of the day, and when they asked if I wanted to press charges (technically it was theft and damage of property), the collector waved a 20 Euro note for "cleaning costs", which I took instead. I felt that was punishment enough.
Then again, there is plenty of bottle collectors who are rather polite, and humbly ask if you're done with the bottle and if they can have it. And a lot of people also gather together their empty bottles in a pile when they see a collector approaching.