And what is the difference between the USA and Germany. In Germany it is also prohibited and the site is subject to a sanction.
There is a difference, though.
Blacklisted Countries as per the gambling service's terms of service vis-à-vis a Country that bans Gambling. In this case, Rollbit seems to have the US on the Blacklist, but Germany is not on the blacklist except for some few games maybe. Everything is in the terms of service and the player should know this.
Now, whether Rollbit service is illegal in Germany or not is another issue, and you can report to the German Authorities if you feel they are not recognized by the laws in the German jurisdiction and therefore should be sanctioned.