You are wrong about the law in Germany.
As you correctly stated, taking photographs is very unregulated (and wasn’t regulated at all until a few years ago) in Germany. It’s only illegal to take photos in the places you listed. (Actually it’s not so much about specific places. The photos have to invade a private area of life, though that usually means places like the home, hotel rooms or restrooms are off limits.) This barrier is actually relatively high, not everything is immediately an invasion of a private area of life, even if it might seem so. A normal shot of you hanging out in a conference room with hundreds other people? Not an invasion of a private area of life.
This also means that privately owned but public spaces are not included there, like, say, conference halls or train stations or shopping malls and similar such places. As you also correctly stated, the owner can disallow photographs, though by default (i.e. if the owner doesn’t have any rules) taking photographs is perfectly ok.
The issue is publishing photographs. There the barriers are much, much higher in Germany. If there are people in it, the default is that you are not allowed to publish them unless every person in the photograph agrees. Now, as always with the law, there are (of course) exceptions (e.g. if people are only incidental to the photograph and not the main subject, say if you take a photo of a building and there happen to be a few people in front of it), though none of them apply in this case. Publishing this photo wouldn’t be legal in Germany (taking it would), though it is important to note that in Germany those depicted in the photo would have to come forward and press charges. Otherwise the police is not allowed to investigate or charge people. (You could say that the default assumption of the law is that everyone in published photographs agreed to it and if you don’t it’s on you to come forward.)
However, that is not the case in the US where both taking and publishing this photo is perfectly legal. So I do not really see the relevance of this discussion.