I live in Cologne and hadn't heard a thing about this until today.
The train station is not particularly nice, but neither is it a red light district. In fact, it's directly next to the cathedral and main tourist area.
I didn't get the impression this had been hushed up, more that it had taken a few days for the women to come forward and for the police to put it all together.
Germany certainly does have rules against large gatherings, to whoever said it didn't!
There was a very large right wing demonstration and many counter demonstrations here a couple of months ago. It was very strongly policed after a demo got out of hand in 2014, when the police underestimated numbers attending.
I have no idea what went on on NYE but to suggest that Germany allows or is more likely to tolerate this sort if thing is in my view incorrect.