That reminds me when I was in Schleswig (lovely town) on my way to Denmark. And I wanted to pay for 1 day stay, lady that had (a really nice) apartment said that unfortunately her terminal doesn't accept international debit/credit cards (wild! and probably not true) and I had to hunt for an ATM machine to get some cash. It was 13 years ago, I remember I was so surprised that the west is so far back behind us.
Berlin is very different from the rest of Germany.
But not how you think, in this instance. I think Berlin has more places of questionable legality which require cash for presumably tax avoidance reasons.