Newbie question: is it really voluntary? When I was in Germany last year I used the toilets at a bar where I was a customer, and not understanding the etiquette/signs, I walked out without paying. But the "creepy guy sitting alone at a table right outside the toilets" started yelling at me loudly about Geld... at which point I understood he was the cleaner and tipped 2 Euro.

(I'm approaching this as an Australian, where public toilets and toilets in shopping malls / department stores are always free. But I don't mind a user-pays system.)